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

llvm-svn: 290358