The usat instruction is available from ARMv6, so there's no need to
check for the ARMv7 feature before using it. ARMv6 is the oldest
supported architecture in V8.

Correcting this allows the removal of a special case for predictable
code size.

BUG=

Review-Url: https://codereview.chromium.org/1974903002
Cr-Commit-Position: refs/heads/master@{#36218}