[RISCV] Add support for custom instructions for Sifive S76.

Support for below instruction is added
1. CFLUSH.D.L1
2. CDISCARD.D.L1
3. CEASE

Additionally, Zihintpause extension is added to sifive s76 for pause
instruction.

Spec - https://sifive.cdn.prismic.io/sifive/767804da-53b2-4893-97d5-b7c030ae0a94_s76mc_core_complex_manual_21G3.pdf

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D153370