x86: Add `Avoid_STOSB` tunable to allow NT memset without ERMS

The goal of this flag is to allow targets which don't prefer/have ERMS
to still access the non-temporal memset implementation.

There are 4 cases for tuning memset:
    1) `Avoid_STOSB && Avoid_Non_Temporal_Memset`
        - Memset with temporal stores
    2) `Avoid_STOSB && !Avoid_Non_Temporal_Memset`
        - Memset with temporal/non-temporal stores. Non-temporal path
          goes through `rep stosb` path. We accomplish this by setting
          `x86_rep_stosb_threshold` to
          `x86_memset_non_temporal_threshold`.
    3) `!Avoid_STOSB && Avoid_Non_Temporal_Memset`
        - Memset with temporal stores/`rep stosb`
    3) `!Avoid_STOSB && !Avoid_Non_Temporal_Memset`
        - Memset with temporal stores/`rep stosb`/non-temporal stores.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>