libc: Access _sigintr more efficiently.

The variable _sigintr is not exported via the version script; therefore,
tell the compiler that no indirection (to allow interposition) is needed.

lib/libc/gen/siginterrupt.c

@@ -46,7 +46,7 @@ int
 siginterrupt(sig, flag)
 	int sig, flag;
 {
-	extern sigset_t _sigintr;
+	extern sigset_t _sigintr __hidden;
 	struct sigaction sa;
 	int ret;
 

lib/libc/gen/signal.c

@@ -41,7 +41,7 @@ __FBSDID("$FreeBSD$");
 #include "un-namespace.h"
 #include "libc_private.h"
 
-sigset_t _sigintr;		/* shared with siginterrupt */
+sigset_t _sigintr __hidden;	/* shared with siginterrupt */
 
 sig_t
 signal(s, a)