sparc64: fix sparse "Should it be static?" warnings in signal32.c

Fix following warnings:
signal32.c:140:6: warning: symbol 'do_sigreturn32' was not declared. Should it be static?
signal32.c:230:17: warning: symbol 'do_rt_sigreturn32' was not declared. Should it be static?
signal32.c:729:6: warning: symbol 'do_signal32' was not declared. Should it be static?
signal32.c:773:16: warning: symbol 'do_sys32_sigstack' was not declared. Should it be static?

Add proper prototypes and drop local prototype

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>