[RISCV][NFC] Deduplicate Atomic Intrinsic Definitions

Summary:
This is a slight cleanup, to use multiclasses to avoid the duplication between
the different atomic intrinsic definitions. The produced intrinsics are
unchanged, they're just generated in a more succinct way.

Reviewers: asb, luismarques, jrtc27

Reviewed By: luismarques, jrtc27

Subscribers: Jim, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, jfb, PkmX, jocewei, psnobl, benna, s.egerton, pzheng, sameer.abuasal, apazos, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71777