[Target][XCOFF] Correctly halt when mixing AIX or XCOFF with ppc64le

The code to prevent using `PPCXCOFFMCAsmInfo` with little-endian targets
used an incorrect check. Also, there does not appear to be sufficient
earlier checking to prevent failing this check, so the check here is
upgraded to be a `report_fatal_error`.

`PPCAIXAsmPrinter` was also missing a check against use with
little-endian targets. This patch adds such a check in.