[compiler-rt][CMAKE] Only add cmake link flags in standalone build

Only add CMAKE_EXE_LINKER_FLAGS when in a standalone bulid.
Or else CMAKE_EXE_LINKER_FLAGS contains flags for build compiler of Clang/llvm.
This might not be the same as what the COMPILER_RT_TEST_COMPILER supports.
eg: the build compiler use lld linker and we use it to build clang with
default ld linker then to be tested clang will complain about lld
options like --color-diagnostics.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D78373