[Clang][LoongArch] Add intrinsic for csrrd, csrwr and csrxchg

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?h=v6.0&id=4fe89d07dcc2804c8b562f6c7896a45643d34b2f#n232

Differential Revision: https://reviews.llvm.org/D139288