[compiler-rt] CMake build: if LLVM_BUILD_32_BITS is on, then -m32 is added to...

[compiler-rt] CMake build: if LLVM_BUILD_32_BITS is on, then -m32 is added to llvm definitions. Let us override this by explicitly adding -m64 compiler flag when building compiler-rt libraries for x86_64 target

llvm-svn: 161415