powerpc[64]: fix missing ctr and xer regs in syscall asm clobberlists

the ctr and xer special registers are call-clobbered and
syscall-clobbered. failure to include them in the clobber list may
result in wrong code that attempts to use a value which is no longer
present in the register after the syscall. this has been reported to
manifest newly with gcc 15.