[AArch64] Fix wrong AArch64Subtarget construction. (#172942)
The AArch64Subtarget construction was wrong for two reasons: firstly,
createMCSubtargetInfo() does not create an AArch64Subtarget object, and
secondly, the target CPU and features were left blank. This has been
benign so far since no methods were called that depended on this, but it
is undefined for the first reason, and creating the subtarget info in a
state that the user did not request for the second reason. This commit
fixes both issues.