[TSan] Adjust `TSAN_RTL_CFLAGS` before it gets copied

Add `COMPILER_RT_LIBDISPATCH_CFLAGS` to `TSAN_RTL_CFLAGS` before it gets
duplicated to `TSAN_RTL_DYNAMIC_CFLAGS` so both versions have the
necessary flags.

Reviewed By: wrotki, rsundahl

Differential Revision: https://reviews.llvm.org/D137183