use @@@ instead of @@ in __sym_default

Using
    .symver foo,foo@@VER
causes foo and foo@@VER to be output to the .o file. This requires foo
to be weak since the linker handles foo@@VER as foo.

Using
    .symver foo,foo@@@VER
causes just foo@@ver to be output and avoid the need for making foo
weak. It also reduces the constraint on how exactly a linker has to
handle foo and foo@@VER being present.

Submitted by:	Rafael Espíndola
Reviewed by:	dim, kib
Differential Revision:	https://reviews.freebsd.org/D11653

lib/libc/sys/openat.c

@@ -43,7 +43,6 @@ __sym_compat(openat, __impl_openat, FBSD_1.1);
 __weak_reference(openat, __impl_openat);
 __sym_default(openat, openat, FBSD_1.2);
 
-#pragma weak openat
 int
 openat(int fd, const char *path, int flags, ...)
 {

lib/libc/sys/setcontext.c

@@ -42,7 +42,6 @@ __sym_compat(setcontext, __impl_setcontext, FBSD_1.0);
 __weak_reference(setcontext, __impl_setcontext);
 __sym_default(setcontext, setcontext, FBSD_1.2);
 
-#pragma weak setcontext
 int
 setcontext(const ucontext_t *uc)
 {

lib/libc/sys/swapcontext.c

@@ -43,7 +43,6 @@ __sym_compat(swapcontext, __impl_swapcontext, FBSD_1.0);
 __weak_reference(swapcontext, __impl_swapcontext);
 __sym_default(swapcontext, swapcontext, FBSD_1.2);
 
-#pragma weak swapcontext
 int
 swapcontext(ucontext_t *oucp, const ucontext_t *ucp)
 {

sys/sys/cdefs.h

@@ -551,7 +551,7 @@
 #define	__sym_compat(sym,impl,verid)	\
 	__asm__(".symver " #impl ", " #sym "@" #verid)
 #define	__sym_default(sym,impl,verid)	\
-	__asm__(".symver " #impl ", " #sym "@@" #verid)
+	__asm__(".symver " #impl ", " #sym "@@@" #verid)
 #else
 #define	__weak_reference(sym,alias)	\
 	__asm__(".weak alias");		\
@@ -563,7 +563,7 @@
 #define	__sym_compat(sym,impl,verid)	\
 	__asm__(".symver impl, sym@verid")
 #define	__sym_default(impl,sym,verid)	\
-	__asm__(".symver impl, sym@@verid")
+	__asm__(".symver impl, sym@@@verid")
 #endif	/* __STDC__ */
 #endif	/* __GNUC__ || __INTEL_COMPILER */