Refined HasAtLeast() for x86 and x86_64.

Rationale:
This refinement is needed once we start passing in more
architecture specific flags on x86_* builds. This CL
also makes the arm version a bit more readable.

Test: instruction_set_features_test
Bug: 63585550

Change-Id: Ia6fe47654141feaf1899da225abaee8a5ea8249a