implement __sigsetjmp()

__sigsetjmp() was stubbed, but we already had sigsetjmp() and __sigsetjmp()
should just be exactly the same.

After this patch, the python3 app no longer prints an annoying stub warning
at startup. control-C now also works, but weirdly - the control-C doesn't
take effect until pressing enter. I don't know why. That's an unrelated bug.

Fixes #645.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

libc/arch/aarch64/setjmp/sigsetjmp.s

@@ -4,10 +4,13 @@
  * BSD license as described in the LICENSE file in the top-level directory.
  */
 
+.global __sigsetjmp
 .global _sigsetjmp
 .global sigsetjmp
+.type __sigsetjmp,@function
 .type _sigsetjmp,@function
 .type sigsetjmp,@function
+__sigsetjmp:
 _sigsetjmp:
 sigsetjmp:
         wfi

libc/arch/x64/setjmp/sigsetjmp.s

@@ -1,6 +1,9 @@
 /* Copyright 2011-2012 Nicholas J. Kain, licensed under standard MIT license */
 .global sigsetjmp
+.global __sigsetjmp
 .type sigsetjmp,@function
+.type __sigsetjmp,@function
+__sigsetjmp:
 sigsetjmp:
 	andl %esi,%esi
 	movq %rsi,64(%rdi)

libc/signal.cc

@@ -627,12 +627,6 @@ int sigaltstack(const stack_t *ss, stack_t *oss)
     return 0;
 }
 
-extern "C" int __sigsetjmp(sigjmp_buf env, int savemask)
-{
-    WARN_STUBBED();
-    return 0;
-}
-
 extern "C" int signalfd(int fd, const sigset_t *mask, int flags)
 {
     WARN_STUBBED();