sigaction: wrap sa_restorer in #ifdef SA_RESTORER consistently

sigaction: wrap sa_restorer in #ifdef SA_RESTORER consistently

Wrap sa_restorer member definitions in #ifdef SA_RESTORER to be
consistent with their use.
If an architecture does not provide sa_restorer members but still
defines SA_RESTORER macro, the latter has to be explicitly undefined.

This change fixes compilation failures like this one:
signal.c: In function 'decode_old_sigaction':
signal.c:631:21: error: 'struct old_sigaction' has no member named 'sa_restorer'
signal.c: In function 'decode_new_sigaction':
signal.c:1224:21: error: 'struct new_sigaction' has no member named 'sa_restorer'

* signal.c (struct old_sigaction, struct old_sigaction32,
struct new_sigaction, struct new_sigaction32):
Wrap sa_restorer member in #ifdef SA_RESTORER.
(decode_old_sigaction, decode_new_sigaction):
Wrap use of sa32.sa_restorer in #ifdef SA_RESTORER.

Signed-off-by: Vicente Olivert Riera <vincent.riera@imgtec.com>