Expose symbols on macos in the xplat pytorch stack

Summary:
X-link: pytorch/pytorch#150487

X-link: pytorch/executorch#9819

Had to revert D71321310 because it affected way too many targets and build sizes.

These changes should expose just enough symbols to be buildable in arvr mode on macOS. Could potentially make narrow it down even more by avoiding eg `get_pt_compiler_flags`

Reviewed By: drisspg

Differential Revision: D72255474

fbshipit-source-id: a196d32526ef826be3baf6ab53e9e0949718bec1

Author: stepanhruda

folly/defs.bzl

@@ -166,7 +166,7 @@ def folly_xplat_library(
             "ovr_config//os:android": FBANDROID_CXXFLAGS,
             "ovr_config//os:iphoneos": CLANG_CXX_FLAGS,
             # TODO: Why iphoneos and macos are not marked as clang compilers?
-            "ovr_config//os:macos": CLANG_CXX_FLAGS,
+            "ovr_config//os:macos": CLANG_CXX_FLAGS + ["-fvisibility=default"],
         }) + select({
             "DEFAULT": [],
             "ovr_config//os:windows-cl": WINDOWS_MSVC_CXXFLAGS,