target/ppc: Rename TBL to TB on 64-bit

target/ppc: Rename TBL to TB on 64-bit

From the earliest PowerPC ISA, TBR (later SPR) 268 has been called TB
and accessed with mftb instruction. The problem is that TB is the name
of the 64-bit register, and 32-bit implementations can only read the
lower half with one instruction, so 268 has also been called TBL and
it does only read TBL on 32-bit.

Change SPR 268 to be called TB on 64-bit implementations.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>