[libunwind][CMake] Treat S files as C to work around CMake bug.

The OSX_ARCHITECTURES property is supposed to add the -arch flag when
targeting Apple platforms. However, due to a bug in CMake
(https://github.com/Kitware/CMake/blob/master/Source/cmLocalGenerator.cxx#L1780),
this does not apply to assembly files. This results in a linker error
when trying to build libunwind for i386 on MacOS.

rdar://59642189

Author: Bigcheese

libunwind/src/CMakeLists.txt

@@ -24,9 +24,11 @@ set(LIBUNWIND_ASM_SOURCES
     UnwindRegistersRestore.S
     UnwindRegistersSave.S
     )
-if (MINGW)
+if (MINGW OR APPLE)
   # CMake doesn't build assembly sources for windows/gnu targets properly
   # (up to current CMake, 3.16), so treat them as C files.
+  # Additionally, CMake ignores OSX_ARCHITECTURE for ASM files when targeting
+  # Apple platforms.
   set_source_files_properties(${LIBUNWIND_ASM_SOURCES}
                               PROPERTIES
                                 LANGUAGE C)