[builtins] Do not set hidden attribute on Android

[kongyi]

Summary

Bionic libc relies on an old libgcc behaviour which does not set hidden visibility attribute. Keep exporting these symbols on Android for compatibility.

[rprichard]

Keep exporting these symbols on Android for compatibility.

For posterity, I think we want the builtins symbols not-hidden so that they can be exported from libc.so and libm.so on 32-bit targets, e.g.:

• for arm32: many __aeabi_* libc.so functions, __fixunsdfdi in libm.so, ...
• for x86: __divdi3, __popcountsi2, __udivdi3, __umoddi3

The platform continues to export these builtins for backwards compatibility with old binaries.

compiler-rt/trunk/lib/builtins/CMakeLists.txt

else ()
 
   append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 BUILTIN_CFLAGS)
 
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.
   if(COMPILER_RT_STANDALONE_BUILD)
     append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC BUILTIN_CFLAGS)
     append_list_if(COMPILER_RT_HAS_FNO_BUILTIN_FLAG -fno-builtin BUILTIN_CFLAGS)
+    if(NOT ANDROID)
       append_list_if(COMPILER_RT_HAS_VISIBILITY_HIDDEN_FLAG -fvisibility=hidden BUILTIN_CFLAGS)
+    endif()
     if(NOT COMPILER_RT_DEBUG)
       append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fomit-frame-pointer BUILTIN_CFLAGS)
     endif()
   endif()
 
   set(BUILTIN_DEFS "")
 
+  if(NOT ANDROID)
     append_list_if(COMPILER_RT_HAS_VISIBILITY_HIDDEN_FLAG VISIBILITY_HIDDEN BUILTIN_DEFS)
+  endif()
 
   foreach (arch ${BUILTIN_SUPPORTED_ARCH})
     if (CAN_TARGET_${arch})
       # NOTE: some architectures (e.g. i386) have multiple names.  Ensure that
       # we catch them all.
       set(_arch ${arch})
       if("${arch}" STREQUAL "armv6m")
         set(_arch "arm|armv6m")