[RISCV] Add target specific loop unrolling and peeling preferences

Both these preference helper functions have initial support with
this change. The loop unrolling preferences are set with initial
settings to control thresholds, size and attributes of loops to
unroll with some tuning done.  The peeling preferences may need
some tuning as well as the initial support looks much like what
other architectures utilize.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D113798