Repeated names for architecture, aarch64 and arm64, needed in XFail list

Hi llvm-dev@,

I've run into a situation where an architecture has two different
names. While editing a debug-info test recently, I needed to XFail it
for aarch64 [0]. However, it then turned out that when building on
Apple M1s their target architecture is spelt "arm64", which wasn't
caught by the XFail list, leading to some unexpected failures [1].

Adding another name to the XFail list isn't a big deal, but having two
names for one thing isn't ideal. Is this is a known limitation, or is
there a better approach to xfailing things for aarch64?.

[0] e05c10380ce7 and 4fd29e4fd3c

XFAIL doesn't do anything sophisticated, it's just a substring match
on the triple. That's why it didn't work.

I ran into something similar at one point, though. There's a test
somewhere (which I'm too lazy to look up right now) that has both
'arm' and 'arm64' triples in it; and this doesn't work if you have
enabled ARM but not AARCH64.

Moving the arm64 RUN line to an aarch64-equivalent test... didn't work.
Somewhere in the commit history you can see my attempt to deal with it
and the fairly quick revert. The problem wasn't annoying enough to be
worth digging deeper, but it would be nice to know what's going on.