Add helper macros to hide some boring repeatable ceremonies to define
ifuncs on x86.

Also keep helpers to define 'pseudo-ifuncs' which are emulated by the
indirect jmp.

Reviewed by: jhb (previous version, as part of the larger patch)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D13838