target/ppc: Rename registers to match ISA

target/ppc: Rename registers to match ISA

Several registers have names that don't match the ISA (or convention
with other QEMU PPC registers), making them unintuitive to use with
GDB.

Fortunately most of these registers are obscure and/or have not been
correctly implemented in the gdb server (e.g., DEC, TB, CFAR), so risk
of breaking users should be low.

QEMU should follow the ISA for register name convention (where there is
no established GDB name).

Acked-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>