powerpc/64e: Clarify register saves and clears with {SAVE,ZEROIZE}_GPRS

The common interrupt handler prologue macro and the bad_stack
trampolines include consecutive sequences of register saves, and some
register clears. Neaten such instances by expanding use of the SAVE_GPRS
macro and employing the ZEROIZE_GPR macro when appropriate.

Also simplify an invocation of SAVE_GPRS targetting all non-volatile
registers to SAVE_NVGPRS.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220921065605.1051927-7-rmclure@linux.ibm.com