ppc: Define SETFIELD for the ppc target

ppc: Define SETFIELD for the ppc target

It keeps repeating, move it to the header. This uses __builtin_ffsll() to
allow using the macros in #define.

This is not using the QEMU's FIELD macros as this would require changing
all such macros found in skiboot (the PPC PowerNV firmware).

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220628080544.1509428-1-aik@ozlabs.ru>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>