prelude: Add fat_mac_linux platform to os lookup platforms

Summary: Currently this just returns `linux` which is... completely wrong

Reviewed By: dtolnay

Differential Revision: D73008565

fbshipit-source-id: c1407790d93db6874823a327043df0b0bc1ebc32

Author: JakobDegen

Diff for: prelude/os_lookup/defs.bzl

@@ -8,6 +8,7 @@
 load("@prelude//decls:core_rules.bzl", "PlatformExePlatform", "TargetCpuType")
 
 Os = enum(
+    "fat_mac_linux",
     "freebsd",
     "unknown",
     *PlatformExePlatform

Diff for: prelude/os_lookup/targets/BUCK

@@ -13,9 +13,14 @@ os_lookup(
         "config//cpu:x86_64": "x86_64",
     }),
     os = select({
+        # FIXME(JakobDegen): No default
         "DEFAULT": "linux",
+        "config//os:linux": "linux",
         "config//os:macos": "macos",
         "config//os:windows": "windows",
+        # FIXME(JakobDegen): We assuem fat means fat mac linux, but that might not be true in the
+        # future.
+        "prelude//platforms:fat_platform_enabled": "fat_mac_linux",
     }),
     script = select({
         "DEFAULT": "sh",