[RISCV] Add preferred function and loop alignment RISCVSubtarget. NFC

These seem like properties we will want to adjust based on -mtune.
Move them to subtarget like is done on ARM and AArch64. Don't add
any overrides yet.

Note there's a slight change here. We are now passing Align(1) for
preferred function alignment where we previously passed the minimum
alignment. As far as I could tell, it will be maxed with min when
it used so this should be ok.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D144048