On further reading of ISA manual it appears gas should have been
treating mftb and mftbu as extended mnemonics for mfspr, for ISA 2.03
and later.
opcodes/
* ppc-opc.c (powerpc_opcodes): Make mftb* generate mfspr for
power4 and later.
gas/testsuite/
* gas/ppc/a2.d: Update for mftb change.
* gas/ppc/476.d: Likewise.