Use struct sigcontext instead of struct sigcontext_struct

* configure.ac (AC_CHECK_TYPES): Remove struct sigcontext_struct.
* signal.c (sys_sigreturn) [S390 || S390X || POWERPC || ALPHA): Replace
struct sigcontext_struct with struct sigcontext.

Reported by John Spencer.
diff --git a/signal.c b/signal.c
index 3411ddd..3a11b8a 100644
--- a/signal.c
+++ b/signal.c
@@ -827,7 +827,7 @@
 #elif defined(S390) || defined(S390X)
 	if (entering(tcp)) {
 		long usp;
-		struct sigcontext_struct sc;
+		struct sigcontext sc;
 		if (upeek(tcp->pid, PT_GPR15, &usp) < 0)
 			return 0;
 		if (umove(tcp, usp + __SIGNAL_FRAMESIZE, &sc) < 0)
@@ -921,7 +921,7 @@
 #elif defined(POWERPC)
 	if (entering(tcp)) {
 		long esp;
-		struct sigcontext_struct sc;
+		struct sigcontext sc;
 
 		esp = ppc_regs.gpr[1];
 
@@ -951,7 +951,7 @@
 #elif defined(ALPHA)
 	if (entering(tcp)) {
 		long fp;
-		struct sigcontext_struct sc;
+		struct sigcontext sc;
 		if (upeek(tcp->pid, REG_FP, &fp) < 0)
 			return 0;
 		if (umove(tcp, fp, &sc) < 0)