[Clang][LoongArch] Add intrinsic for iocsrrd and iocsrwr

These intrinsics are required by Linux [1].

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/loongarch/include/asm/loongarch.h#n240

Differential Revision: https://reviews.llvm.org/D139612