[CompilerRT] Don't pass global compile test flags in non-standalone build

In a build with -DLLVM_ENABLE_LTO=Thin:

$ ninja TSanitizer-x86_64-Test-Nolibc
[1/1] Generating Sanitizer-x86_64-Test-Nolibc
FAILED: projects/compiler-rt/lib/sanitizer_common/tests/Sanitizer-x86_64-Test-Nolibc
sanitizer_nolibc_test_main.x86_64.o: file not recognized: file format not recognized

because -flto=thin is getting passed to the clang_compile step.

For non-standalone builds, global compilation flags shouldn't be passed to compiler-rt tests, only the flags the test specifies.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D84466