c18n: Override libsys's sigaction to directly call RTLD hook

Previously libsys's sigaction calls a libc function which in turn calls
the RTLD hook. This is broken but was undetected because c18n was
accidentally not enabled in the c18n-version of libsys.

lib/libc/include/libc_private.h

@@ -396,7 +396,7 @@ int __strerror_rl(int errnum, char *strerrbuf, size_t buflen,
 
 #ifdef CHERI_LIB_C18N
 __pid_t		__sys_rfork(int);
-int	sigaction_c18n(int, const struct sigaction *, struct sigaction *);
+int	_rtld_sigaction(int, const struct sigaction *, struct sigaction *);
 #endif
 
 #endif /* _LIBC_PRIVATE_H_ */

lib/libc/sys/sigaction.c

@@ -35,19 +35,6 @@
 
 __weak_reference(__sys_sigaction, __sigaction);
 __weak_reference(sigaction, __libc_sigaction);
-#ifdef CHERI_LIB_C18N
-/*
- * This weak symbol will always be resolved at runtime.
- */
-#pragma weak _rtld_sigaction
-int _rtld_sigaction(int, const struct sigaction *, struct sigaction *);
-
-int
-sigaction_c18n(int sig, const struct sigaction *act, struct sigaction *oact)
-{
-	return (_rtld_sigaction(sig, act, oact));
-}
-#endif
 
 #pragma weak sigaction
 int

lib/libsys/interposing_table.c

@@ -57,7 +57,7 @@ static interpos_func_t __libsys_interposing[INTERPOS_MAX] = {
 	SLOT(sendto, __sys_sendto),
 	SLOT(setcontext, __sys_setcontext),
 #ifdef CHERI_LIB_C18N
-	SLOT(sigaction, sigaction_c18n),
+	SLOT(sigaction, _rtld_sigaction),
 #else
 	SLOT(sigaction, __sys_sigaction),
 #endif