[RISCV] Don't add -unaligned-scalar-mem to target features by default.

Only pass it +unaligned-scalar-mem/-unaligned-scalar-mem if the
user has passed one of the alignment options.

This allows us to add unaligned-scalar-mem as a feature on CPUs
that support it.

Reviewed By: wangpc

Differential Revision: https://reviews.llvm.org/D159145