powerpc: Don't use thread struct for saving SRR0/1 on syscall.

CR0 can be saved later, and CTR can also be used for saving.

Keep SRR1 in r9 and stash SRR0 in CTR, this avoids using thread_struct
in memory for that.

Saves 3 cycles (ie 1%) in null_syscall selftest on 8xx.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b94c3bc03bac9431fec2dadb686384c481889422.1580470483.git.christophe.leroy@c-s.fr