[RISCV] Add Clang frontend support for Bitmanip extension

Summary: This adds the __riscv_bitmanip macro and the 'b' target feature to enable it.

Reviewers: asb, simoncook, lewis-revill, PaoloS, lenary

Reviewed By: lenary

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71553