Make clang_rt optional

This patch replaces the `assert(false)` in `template("clang_lib")` to silently skip clang_rt on CPUs where it is not supported.
See #93 (comment)

Author: bblanchon

patches/clang_rt.patch

@@ -0,0 +1,55 @@
+diff --git a/config/clang/BUILD.gn b/config/clang/BUILD.gn
+index c38670a..5911f4b 100644
+--- a/config/clang/BUILD.gn
++++ b/config/clang/BUILD.gn
+@@ -183,7 +183,7 @@ template("clang_lib") {
+         } else if (current_cpu == "arm64") {
+           _suffix = "-aarch64"
+         } else {
+-          assert(false)  # Unhandled cpu type
++          # Unhandled cpu type
+         }
+       } else if (is_apple) {
+         _dir = "darwin"
+@@ -205,7 +205,7 @@ template("clang_lib") {
+         } else if (current_cpu == "s390x") {
+           _dir = "s390x-unknown-linux-gnu"
+         } else {
+-          assert(false)  # Unhandled cpu type
++          # Unhandled cpu type
+         }
+       } else if (is_fuchsia) {
+         if (current_cpu == "x64") {
+@@ -213,7 +213,7 @@ template("clang_lib") {
+         } else if (current_cpu == "arm64") {
+           _dir = "aarch64-unknown-fuchsia"
+         } else {
+-          assert(false)  # Unhandled cpu type
++          # Unhandled cpu type
+         }
+       } else if (is_android) {
+         _dir = "linux"
+@@ -228,15 +228,20 @@ template("clang_lib") {
+         } else if (current_cpu == "riscv64") {
+           _suffix = "-riscv64-android"
+         } else {
+-          assert(false)  # Unhandled cpu type
++          # Unhandled cpu type
+         }
+       } else {
+-        assert(false)  # Unhandled target platform
++        # Unhandled target platform
+       }
+ 
+       _clang_lib_dir = "$clang_base_path/lib/clang/$clang_version/lib"
+       _lib_file = "${_prefix}clang_rt.${_libname}${_suffix}.${_ext}"
+-      libs = [ "$_clang_lib_dir/$_dir/$_lib_file" ]
++      if (_dir != "") {
++        libs = [ "$_clang_lib_dir/$_dir/$_lib_file" ]
++      } else {
++        print("WARNING: No clang runtime library for $target_os/$current_cpu")
++        print("$_clang_lib_dir/????/$_lib_file")
++      }
+     }
+   }
+ }

steps/03-patch.sh

@@ -26,6 +26,7 @@ case "$BUILD_TYPE" in
 esac
 
 apply_patch "$PATCHES/public_headers.patch"
+apply_patch "$PATCHES/clang_rt.patch" build
 
 [ "$ENABLE_V8" == "true" ] && apply_patch "$PATCHES/v8/pdfium.patch"