Port subr_physmem to userspace and add tests

These give us some confidience we haven't broken anything in early
boot code that may be running before the console.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34691

(cherry picked from commit 8c99dfed541263bd51cae943684bcc13768c2a36)

Disable the physmem test for now

It fails to build on at least i386

(cherry picked from commit d8819d88af529974bac1fe49b713dacf1f3d7239)

Enable subr_physmem_test on supported architectures

Only build where it's supported.

While here add support for amd64 to help with testing.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 41e6d2091ce5653d4ace3f0b9cbde1375d458c67)