The ARM gas driver previously enabled NEON for ARMv7 and up, and a
handful of other extensions for ARMv8 and up (although only if the
architecture version was a part of the triple; the -march flag was
not honored). Neither of these are actually guaranteed, and the
behavior does not match the integrated assembler.
Note that I've elected to always pass an explicit -march flag to gas
if the user has not specified one. I'm not certain if Clang's default
ARM version matches GNU's, so being explicit allows us to keep the
same behavior if that isn't the case. This also makes it clear that
the correct architecture is passed to gas based on -mcpu.