change jmp_buf to share an underlying type and struct tag with sigjmp_buf

this is necessary to meet the C++ ABI target. alternatives were
considered to avoid the size increase for non-sig jmp_buf objects, but
they seemed to have worse properties. moreover, the relative size
increase is only extreme on x86[_64]; one way of interpreting this is
that, if the size increase from this patch makes jmp_buf use too much
memory, then the program was already using too much memory when built
for non-x86 archs.

src/signal/siglongjmp.c

@@ -7,5 +7,5 @@
 _Noreturn void siglongjmp(sigjmp_buf buf, int ret)
 {
 	if (buf->__fl) __restore_sigs(buf->__ss);
-	longjmp(buf->__jb, ret);
+	longjmp(buf, ret);
 }

src/signal/sigsetjmp.c

@@ -11,5 +11,5 @@ int sigsetjmp(sigjmp_buf buf, int save)
 {
 	if ((buf->__fl = save))
 		pthread_sigmask(SIG_SETMASK, 0, (sigset_t *)buf->__ss);
-	return setjmp(buf->__jb);
+	return setjmp(buf);
 }