MFC r268880:

MFC r268880:
Allow mappings of memory not previously direct-mapped by the kernel when
calling mmap on /dev/mem and add a handler for the possible userland
machine checks that may result. Remove some pointless and wrong copy/paste
that has been in here for a decade as well.

This results in a /dev/mem with identical semantics to the x86 version.

git-svn-id: svn://svn.freebsd.org/base/stable/10@271113 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f