[sanitizer] Refactor how assembly files are handled

This renames ASM_TSAN_SYMBOL and ASM_TSAN_SYMBOL_INTERCEPTOR to just ASM_SYMBOL and ASM_SYMBOL_INTERCEPTOR, because they can be useful in more places than just TSan. Also introduce a CMake function to add ASM sources to a target.

Differential Revision: https://reviews.llvm.org/D40143

llvm-svn: 319339