Add brk and sbrk

A few things need these to build.
Implement pmap_ts_referenced

We only handle the non-superpage case but it removes a panic when
this function gets called.
Add some of the needed symbols from libc
Search for the boot device in loader.efi

This allows us to boot without having this hardcoded or having to
enter it at the mountroot prompt.
Add a partial implementation of pmap_remove_write

This is all I've hit so far, e.g. mounting/unmounting filesystems.
Build boot1.efi for arm64

We can use the boot1 code directly with a few Makefile changes.
Allow the IL bit to be clear in one case

In an exception from the same level when the ISV bit (bit 24) is set
the IL bit (bit 25) may not be set as is the case with QEMU.