[CMake] Explicitly add --target option to compiler flags

Summary:
Much of the non-Darwin build system assumes that COMPILER_RT_DEFAULT_TARGET_TRIPLE is the default target triple for the compiler being used. With clang as your compiler this isn't necessarily true.

To ensure that the rest of the build system behaves as expected this patch adds "--target=${COMPILER_RT_DEFAULT_TARGET_TRIPLE}" to the compiler flags for C, CXX and ASM sources.

Reviewers: compnerd, rengolin, fjricci

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D24156

llvm-svn: 282024