amd64: bring back asm bcmp, shared with memcmp

Turns out clang converts "memcmp(foo, bar, len) == 0" and similar to
bcmp calls.

Reviewed by: emaste (previous version), jhb (previous version)
Differential Revision: https://reviews.freebsd.org/D34673
Approved by: re (gjb)

(cherry picked from commit fbc002cb72d2d9bb435cce99630d5d7da9f59390)
(cherry picked from commit c5890784f6d315c501586b39cf8c0c80b609be5b)