sanitizer_common: Fix build for tests

It turns out that COMPILER_RT_TEST_COMPILER_CFLAGS is actually a string
that is being appended to and not a list.

Therefore, append the thread-safety flags to the string. Because CMake
separates list elements by ';' when turning into a string, also
substitute ';' with ' '.

Reviewed By: hctim

Differential Revision: https://reviews.llvm.org/D105829