Revert "Follow up to e05c103: add aarch64 to test XFails"

This reverts commit 4fd29e4.

There's a report in D95617 that this is failing on what (I think?) is an
aarch64 bot, which should be covered by the XFail list... reverting this
follow-up and the base patch until I work out what's wrong here.

Author: jmorse

llvm/test/DebugInfo/Generic/missing-abstract-variable.ll

@@ -3,7 +3,7 @@
 ; The formal parameter 'b' for Function 'x' when inlined within 'a' is lost on
 ; powerpc64 (and on x86_64 at at least -O2). Presumably this is a SelectionDAG
 ; issue.
-; XFAIL: powerpc64, aarch64
+; XFAIL: powerpc64
 
 ; Build from the following source with clang -O2.