[CMake][compiler-rt][RISCV] Support RISC-V cross-compilation

This seems to be a safe way to ensure that the Compiler-RT test compiler
flags are properly set in all cross-compilation scenarios. Without this
when `BUILTINS_TEST_TARGET_CFLAGS` is set in
`compiler-rt/test/builtins/CMakeLists.txt` the other flags are cleared.

Differential Revision: https://reviews.llvm.org/D92124