Apply mapping protections to preloaded kernel modules on amd64.

With an upcoming change the amd64 kernel will map preloaded files RW
instead of RWX, so the kernel linker must adjust protections
appropriately using pmap_change_prot().

Reviewed by: kib
MFC after: 1 month
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21860