arm: set alignment properly for asm funcs

ARM requires a 4-byte alignment on all ARM code (though this
requirement is relaxed to 2-byte for some THUMB code) and we
should be explicit about that here.

GAS has its own fix for this[1] that forces proper alignment
on any section containing assembled instructions, but this is
not universal: Clang's and other gaslike assemblers lack this
implicit alignment. Whether or not this is considered a bug in
those assemblers, it is better to ask directly for what we want.

[1]: https://sourceware.org/bugzilla/show_bug.cgi?id=12931



Signed-off-by: Sam Edwards <CFSworks@gmail.com>