LLVM_BUILD_32_BITS: Add -m32 with CMAKE_C*_FLAGS. [CMP0056]

With CMP0056, try_compile() uses also CMAKE_EXE_LINKER_FLAG.
It caused mismatch between CMAKE_CXX_FLAGS and CMAKE_EXE_LINKER_FLAGS, to fail to examine CXX_SUPPORTS_CXX11 with -m32.

FYI, before this, try_compile() tries without -m32 regardless of LLVM_BUILD_32_BITS.

llvm-svn: 271871