sparc32: fix sparse warnings in signal_32.c

Fix following warnings:
signal_32.c:62:17: warning: symbol 'do_sigreturn' was not declared. Should it be static?
signal_32.c:126:17: warning: symbol 'do_rt_sigreturn' was not declared. Should it be static?
signal_32.c:344:39: warning: incorrect type in initializer (different address spaces)
signal_32.c:344:39:    expected struct __siginfo_fpu_t [usertype] *fp
signal_32.c:344:39:    got void [noderef] <asn:1>*[assigned] tail
signal_32.c:346:45: warning: incorrect type in argument 2 (different address spaces)
signal_32.c:346:45:    expected struct __siginfo_fpu_t [noderef] [usertype] <asn:1>*fpu
signal_32.c:346:45:    got struct __siginfo_fpu_t [usertype] *fp
signal_32.c:352:41: warning: incorrect type in initializer (different address spaces)
signal_32.c:352:41:    expected struct __siginfo_rwin_t [usertype] *rwp
signal_32.c:352:41:    got void [noderef] <asn:1>*[assigned] tail
signal_32.c:354:48: warning: incorrect type in argument 2 (different address spaces)
signal_32.c:354:48:    expected struct __siginfo_rwin_t [noderef] [usertype] <asn:1>*rwin
signal_32.c:354:48:    got struct __siginfo_rwin_t [usertype] *rwp
signal_32.c:509:6: warning: symbol 'do_notify_resume' was not declared. Should it be static?
signal_32.c:520:16: warning: symbol 'do_sys_sigstack' was not declared. Should it be static?

Add missing prototypes and annotate two variables with __user.

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