[RISCV] Set Fast flag for unaligned memory accesses

The +unaligned-scalar-mem and +unaligned-vector-mem features were added in
D126085 and D149375 respectively to allow subtargets to indicate that
they supported misaligned loads/stores with "sufficient" performance.
This is separate from whether or not the target actually supports
misaligned accesses, which could be determined from Zicclsm.

This patch enables the Fast flag under the assumption that any subtarget
that declares support for +unaligned-*-mem will want to opt into
optimisations that take advantage of misaligned scalar accesses, such as
store merging.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D150771