libc: switch to __weak_symbol for fortified interpose stubs

GCC doesn't like `#pragma weak macro(foo)`, but that's fine; just slap
a __weak_symbol on the definition to more reliably make it weak
anyways.

Reviewed by:	kib (earlier version), markj
Sponsored by:	Klara, Inc.
Sponsored by:	Stormshield
Differential Revision:	https://reviews.freebsd.org/D45976

lib/libc/sys/poll.c

@@ -36,8 +36,7 @@
 
 __weak_reference(__sys_poll, __poll);
 
-#pragma weak __ssp_real(poll)
-int
+int __weak_symbol
 __ssp_real(poll)(struct pollfd pfd[], nfds_t nfds, int timeout)
 {
 	return (INTERPOS_SYS(poll, pfd, nfds, timeout));

lib/libc/sys/ppoll.c

@@ -36,8 +36,7 @@
 
 __weak_reference(__sys_ppoll, __ppoll);
 
-#pragma weak __ssp_real(ppoll)
-int
+int __weak_symbol
 __ssp_real(ppoll)(struct pollfd pfd[], nfds_t nfds,
     const struct timespec *__restrict timeout,
     const sigset_t *__restrict newsigmask)

lib/libc/sys/readv.c

@@ -38,8 +38,7 @@
 
 __weak_reference(__sys_readv, __readv);
 
-#pragma weak readv
-ssize_t
+ssize_t __weak_symbol
 __ssp_real(readv)(int fd, const struct iovec *iov, int iovcnt)
 {
 	return (INTERPOS_SYS(readv, fd, iov, iovcnt));

lib/libc/sys/recvfrom.c

@@ -37,8 +37,7 @@
 
 __weak_reference(__sys_recvfrom, __recvfrom);
 
-#pragma weak recvfrom
-ssize_t
+ssize_t __weak_symbol
 __ssp_real(recvfrom)(int s, void *buf, size_t len, int flags,
     struct sockaddr * __restrict from, socklen_t * __restrict fromlen)
 {

lib/libc/sys/recvmsg.c

@@ -37,8 +37,7 @@
 
 __weak_reference(__sys_recvmsg, __recvmsg);
 
-#pragma weak recvmsg
-ssize_t
+ssize_t __weak_symbol
 __ssp_real(recvmsg)(int s, struct msghdr *msg, int flags)
 {
 	return (INTERPOS_SYS(recvmsg, s, msg, flags));