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.

Provide page allocator as part of bmk-core.

Platforms now share common page allocator code.

add bmk_strtoul() to bmk-core

Add jsmn JSON parser to bmk-core.

One could debate whether we need it in bmk-core or rumprun-base, but
given that it's extremely small and the only external dependency is to
NULL (apart from size_t which I converted to unsigned long), we might
just as well put it into core and see if in-core uses pop up.

MIT licensed, from http://zserge.com/jsmn.html

Don't build kernel-level sources with stack protector.

Technically we should, but since __stack_chk_fail() currently always
comes from libc, things get a bit weird.

Rename librumprun_core to libbmk_core.

It doesn't have anything to do with rumprun, but is at a lower layer of
abstraction, so let's just match the header namespace here.