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 ' '.
COMPILER_RT_TEST_COMPILER_CFLAGS may have escaped ;
could you fix ; of THREAD_SAFETY_FLAGS in a separate var before appending?