aarch64 and arm64 inconsistencies

[jbergstroem]

./configure currently sets host/target_arch as arm64 whereas Makefile will treat it as aarch64 (uname -s doesn't match anything). Suggesting we give Makefile a similar treatment?

/cc @nodejs/build

[rvagg]

I don't mind unifying it a little like we do for x86/ia32 (although even there we're not fully unified thanks mainly to gyp).

@jbergstroem as an aside: both ARCH and DESTCPU are passed for the iojs+release Jenkins job which is why we need to have them explicitly set on the release machines, otherwise it comes out as ARCH="" DESTCPU="" and in Makefile that's read as those variables being set and not up for auto detection. xgene-3 missing both of these env vars lead to failure because Jenkins is configured to use them (which I prefer fwiw).

[jbergstroem]

@rvagg I would personally prefer using them only if set so we can fall back on defaults but don't have a strong opinion about it. Don't break what works (like I did last time), I guess.