Don't issue a warning when ignoring signals

Since we are ignoring signals anyways. This also disables the warning
if `act` is NULL, since we return a valid structure in `oact`.

sigaction: make osa_handler SIG_IGN instead of SIG_DFL.

Latter was accidental. Former is more honest.

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.

Add stub sigpending()

Add compat strong aliases for sigaction/sigprocmask.

These aliases are normally provided by assembly syscall wrappers,
so we do not get them automatically.

Required mostly for configure scripts, as noted by @liuw in
issue #9.

Open-code signal stubs a bit.

Can later stuff some better emulation in there, if need be.