[RISCV] Use default attributes for intrinsics

This switches a large subset of RISC-V intrinsics to use default
attributes (nosync, nofree, nocallback and willreturn).

I didn't reformat a lot of this. I can spend more time on that if
we think it's important.

Reviewed By: nikic, asb, eopXD

Differential Revision: https://reviews.llvm.org/D140360