Add initial ARM support.

A "hello world" works. This means that pthreads, TLS, scheduling
etc. work. There is no I/O support currently (except console).
Adding I/O support requires some fixes to NetBSD (most likely limited
to minor adjustments).

There are two paths to go from here: either add I/O device support,
or fix some structural problems in rumprun which make it assume x86.