Pass -Wa,-mbig-obj in 64-bit mingw builds

COFF has a 2**16 section limit, and on Win64, every COMDAT function
creates at least 3 sections: .text, .pdata, and .xdata. For MSVC, we
enable bigobj on a file-by-file basis, but GCC appears to hit the limit
on different files.

Fixes PR25953

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290358 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rnk

cmake/modules/HandleLLVMOptions.cmake

@@ -232,6 +232,13 @@ if(MSVC)
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /STACK:10000000")
 elseif(MINGW) # FIXME: Also cygwin?
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--stack,16777216")
+
+  # Pass -mbig-obj to mingw gas on Win64. COFF has a 2**16 section limit, and
+  # on Win64, every COMDAT function creates at least 3 sections: .text, .pdata,
+  # and .xdata.
+  if (CMAKE_SIZEOF_VOID_P EQUAL 8)
+    append("-Wa,-mbig-obj" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  endif()
 endif()
 
 if( MSVC )