Fix guard for siginfo_t and pthread_t definition

Fix guard for siginfo_t and pthread_t definition

Commit 8a3b3bb4d7224d419cc1a4af60ccf7e70edc876b changed the guard on
some functions from _POSIX_THREADS to __POSIX_VISIBLE. As a consequence,
some use of siginfo_t and pthread_t became visible under configurations
where _POSIX_THREADS is unset but __POSIX_VISIBLE is. Build then fails
because the definition of those types are still unavailable.

This commit make those type definition visible for __POSIX_VISIBLE
configurations. This requires moving the siginfo_t definition out of the
RTEMS specific definitions in sys/signal.h while still guarding it
against cygwin case.