This makes these targets consistent with ARM/PowerPC in that they
technically accept ABI information via MCTargetOptions::ABIName but they
only implement one ABI (the empty string).
AArch64 is slightly special in that it also accepts "aapcs" and "darwinpcs"
in addition to the empty string. These names are only used in Clang and
have no functional effect on the backend.
X86 is somewhat special in that it's the only target that supports multiple
ABI's but only accepts the empty string. I can define "32", "x32", and "64"
if that's desirable but it's not necessary for functionality.
No corresponding change is required to clang because targets that don't pass
-target-abi get the empty string by default and those that need to pass it
already do so.
Depends on D21069