[RISCV] Set MaxAtomicInlineWidth and MaxAtomicPromoteWidth for RV32/RV64 targets with atomics

Summary: This ensures that libcalls aren't generated when the target supports atomics. Atomics aren't in the base RV32I/RV64I instruction sets, so MaxAtomicInlineWidth and MaxAtomicPromoteWidth are set only when the atomics extension is being targeted. This must be done in setMaxAtomicWidth, as this should be done after handleTargetFeatures has been called.

Reviewers: jfb, jyknight, wmi, asb

Reviewed By: asb

Subscribers: pzheng, MaskRay, s.egerton, lenary, dexonsmith, psnobl, benna, Jim, JohnLLVM, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, kito-cheng, shiva0217, jrtc27, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, lewis-revill, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57450

llvm-svn: 370073