Replace arch_preprocessor_flags with selects in preprocessor_flags

Summary:
Migrate all BUCK file usages of `arch_preprocessor_flags` to `select()` in
`preprocessor_flags`. Architecture mapping:
- `"x86_64"` → `"ovr_config//cpu:x86_64"`
- `"aarch64"` → `"ovr_config//cpu:arm64"`

For targets with existing `preprocessor_flags`, the select is merged via `+`.

Reviewed By: dtolnay

Differential Revision: D92610949

fbshipit-source-id: 3af7311cbfe0992101415ef8615aa1bd5ff7bb98

Author: kkolur76

folly/BUCK

@@ -4797,12 +4797,13 @@ fbcode_target(
     }),
     auto_headers = AutoHeaders.NONE,
     headers = [],
-    arch_preprocessor_flags = {
-        "x86_64": [
+    modular_headers = False,
+    preprocessor_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:x86_64": [
             "-mavx2",
         ],
-    },
-    modular_headers = False,
+    }),
     exported_deps = select({
         "DEFAULT": [],
         "ovr_config//os:linux-arm64": [
@@ -4838,17 +4839,18 @@ fbcode_target(
     }),
     auto_headers = AutoHeaders.NONE,
     headers = [],
-    arch_preprocessor_flags = {
-        "aarch64": [
+    link_whole = True,  # Set link_whole to force linker to use __folly_memset
+    modular_headers = False,
+    preprocessor_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:arm64": [
             "-DFOLLY_MEMSET_IS_MEMSET",
         ],
-        "x86_64": [
+        "ovr_config//cpu:x86_64": [
             "-DFOLLY_MEMSET_IS_MEMSET",
             "-mavx2",
         ],
-    },
-    link_whole = True,  # Set link_whole to force linker to use __folly_memset
-    modular_headers = False,
+    }),
     exported_deps = select({
         "DEFAULT": [],
         "ovr_config//os:linux-arm64": [
@@ -4873,12 +4875,13 @@ fbcode_target(
     }),
     auto_headers = AutoHeaders.NONE,
     headers = [],
-    arch_preprocessor_flags = {
-        "x86_64": [
+    modular_headers = False,
+    preprocessor_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:x86_64": [
             "-mavx2",
         ],
-    },
-    modular_headers = False,
+    }),
     exported_deps = select({
         "DEFAULT": [],
         "ovr_config//os:linux-arm64": [
@@ -4914,18 +4917,19 @@ fbcode_target(
     }),
     auto_headers = AutoHeaders.NONE,
     headers = [],
-    arch_preprocessor_flags = {
-        "aarch64": [
+    link_whole = True,  # Set link_whole to force linker to use __folly_memcpy
+    modular_headers = False,
+    preprocessor_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:arm64": [
             "-DFOLLY_MEMCPY_IS_MEMCPY",
         ],
-        "x86_64": [
+        "ovr_config//cpu:x86_64": [
             "-DFOLLY_MEMCPY_IS_MEMCPY",
             "-mavx2",
             "-march=haswell",
         ],
-    },
-    link_whole = True,  # Set link_whole to force linker to use __folly_memcpy
-    modular_headers = False,
+    }),
     exported_deps = select({
         "DEFAULT": [],
         "ovr_config//os:linux-arm64": [

folly/detail/BUCK

@@ -987,13 +987,14 @@ fbcode_target(
     name = "traponavx512",
     srcs = ["TrapOnAvx512.cpp"],
     headers = ["TrapOnAvx512.h"],
-    arch_preprocessor_flags = {
-        "x86_64": [
+    compiler_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:x86_64": [
             "-msse4.2",
             "-mavx512f",
             "-mavx512vl",
         ],
-    },
+    }),
     deps = [
         "//folly:portability",
     ],

folly/external/aor/BUCK

@@ -34,11 +34,12 @@ fbcode_target(
     headers = [
         "asmdefs.h",
     ],
-    arch_preprocessor_flags = {
-        "aarch64": [
+    preprocessor_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:arm64": [
             "-DFOLLY_MEMCPY_IS_MEMCPY",
         ],
-    },
+    }),
     target_compatible_with = [
         "ovr_config//os:linux-arm64",
     ],
@@ -69,11 +70,12 @@ fbcode_target(
     headers = [
         "asmdefs.h",
     ],
-    arch_preprocessor_flags = {
-        "aarch64": [
+    preprocessor_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:arm64": [
             "-DFOLLY_MEMSET_IS_MEMSET",
         ],
-    },
+    }),
     target_compatible_with = [
         "ovr_config//os:linux-arm64",
     ],

folly/external/fast-crc32/BUCK

@@ -12,13 +12,14 @@ fbcode_target(
     headers = [
         "avx512_crc32c_v8s3x4.h",
     ],
-    arch_preprocessor_flags = {
-        "x86_64": [
+    compiler_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:x86_64": [
             "-mavx512f",
             "-mavx512vl",
             "-mpclmul",
         ],
-    },
+    }),
     exported_deps = [
         "//folly:portability",
     ],
@@ -47,11 +48,12 @@ fbcode_target(
     headers = [
         "neon_crc32c_v3s4x2e_v2.h",
     ],
-    arch_preprocessor_flags = {
-        "aarch64": [
+    compiler_flags = select({
+        "DEFAULT": [],
+        "ovr_config//cpu:arm64": [
             "-march=armv8-a+crypto+crc",
         ],
-    },
+    }),
     deps = [
         "//folly:portability",
         "//folly/system:aux_vector",