sparc64: Don't clibber fixed registers in __multi4.

%g4 and %g5 are fixed registers used by the kernel for the thread
pointer and the per-cpu offset.  Use %o4 and %g7 instead.

Diagnosis by Anthony Yznaga.

Fixes: 1b4af13f ("sparc64: Add __multi3 for gcc 7.x and later.")
Reported-by: Anatoly Pugachev <matorola@gmail.com>
Tested-by: Anatoly Pugachev <matorola@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>