Set PG_G global mapping bit on the trampoline ptes.

Trampoline mappings are better treated as global since they are valid
in all address spaces, even for PTI. pmap_invalidate_range() must work
on global mappings for pti since kernel_pmap invalidations are really
same as for non-PTI.

Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D15052