sim: ppc: fix some Wunused-function warnings

When compiling with --enable-werror and CFLAGS="-O0 -g -Wall", we run into:
...
In file included from src/sim/ppc/cpu.h:251:0,
                 from src/sim/ppc/emul_generic.h:24,
                 from src/sim/ppc/emul_generic.c:24:
src/sim/ppc/cpu.c:76:1: error: 'cpu_create' defined but not used \
  [-Werror=unused-function]
 cpu_create(psim *system,
 ^~~~~~~~~~
...

The function is defined as:
...
INLINE_CPU\
(cpu *)
cpu_create(psim *system,
...
which expands to:
...
static cpu * __attribute__((__unused__))
cpu_create(psim *system,
...

The problem is that gcc does not associate the attribute to the function.
I've filed a PR about this ( PR gcc/100670 ), which may or may not be valid.

Work around/fix this by modifying the INLINE_* definitions in inline.h to move
UNUSED to the start such that we have:
...
__attribute__((__unused__)) static cpu *
cpu_create(psim *system,
...