[POWERPC] powerpc: fix building gdb against asm/ptrace.h

Ulrich Weigand found a bug with the current version of the
asm-powerpc/ptrace.h that prevents building at least the
SPU target version of gdb, since some ptrace opcodes are
not defined.

The problem seems to have originated in the merging of 32 and
64 bit versions of that file, the problem is that some opcodes
are only valid on 64 bit kernels, but are also used by 32 bit
programs, so they can't depends on the __powerpc64__ symbol.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>