Use ifunc to resolve context switching mode on amd64.

Patch removes all checks for pti/pcid/invpcid from the context switch
path. I verified this by looking at the generated code, compiling with
the in-tree clang. The invpcid_works1 trick required inline attribute
for pmap_activate_sw_pcid_pti() to work.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
Differential revision: https://reviews.freebsd.org/D17181