lib/libc/amd64: add archlevel-based simd dispatch framework

Add a framework for selecting from one of multiple implementations
of a function based on amd64 architecture level (cf. amd64 SysV
ABI supplement).

Sponsored by: The FreeBSD Foundation
Approved by: kib
Reviewed by: jrtc27
Differential Revision: https://reviews.freebsd.org/D40693