[builtins] Make sure builtin compile tests respect CMAKE_C_COMPILER_TARGET

Summary:
Since we generate the compiler invocation on our own, we need to
manually add -target if CMAKE_C_COMPILER_TARGET has been specified.

Reviewers: compnerd, beanz

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23823

llvm-svn: 279747