Fix build with gcc.

gcc gave an error because the `char *` parameter doesn't match the
`const char *` in the prototype of regsave().  clang didn't give an
error.  I added the prototype in commit 5301cce.

mach/i386/ncg/mach.c

@@ -140,7 +140,7 @@ f_regsave()
 
 void
 regsave(regstr, off, size)
-	char *regstr;
+	const char *regstr;
 	long off;
 {
 	if (strcmp(regstr, "esi") == 0) {

mach/i86/ncg/mach.c

@@ -137,7 +137,7 @@ f_regsave()
 
 void
 regsave(regstr, off, size)
-	char *regstr;
+	const char *regstr;
 	long off;
 {
 	if (strcmp(regstr, "si") == 0) {

mach/m68020/ncg/mach.c

@@ -184,7 +184,7 @@ f_regsave()
 
 void
 regsave(s,off,size)
-	char *s;
+	const char *s;
 	long off;
 {
 	assert (regnr < 9);