[builtins][LoongArch] Port __clear_cache to LoongArch Linux

There are two failures in the current builtins,
Failed Tests (2):
  Builtins-loongarch64-linux :: clear_cache_test.c
  Builtins-loongarch64-linux :: enable_execute_stack_test.c

It is caused by __clear_cache not being implemented and triggering `abort`.

"__clear_cache" is implemented in the same way as "clear_cache" in gcc (
in gcc/config/loongarch/loongarch.md)

Differential Revision: https://reviews.llvm.org/D136921