Repair portability of sign extension when reading signed ints on boxes

where sizeof(long)==8.  This *was* broken on boxes where signed right
shifts didn't sign-extend, but not elsewhere.  Unfortunately, apart
from the Cray T3E I don't know of such a box, and Guido has so far
refused to buy me any Cray machines for home Python testing <wink>.

More immediately interesting would be if someone could please test
this on *any* sizeof(long)==8 box, to make sure I didn't break it.

Modules/structmodule.c

@@ -653,11 +653,9 @@ bu_int(const char *p, const formatdef *f)
 	do {
 		x = (x<<8) | (*p++ & 0xFF);
 	} while (--i > 0);
-	i = 8*(sizeof(long) - f->size);
-	if (i) {
-		x <<= i;
-		x >>= i;
-	}
+	/* Extend the sign bit. */
+	if (SIZEOF_LONG > f->size)
+		x |= -(x & (1L << (8*f->size - 1)));
 	return PyInt_FromLong(x);
 }
 
@@ -767,11 +765,9 @@ lu_int(const char *p, const formatdef *f)
 	do {
 		x = (x<<8) | (p[--i] & 0xFF);
 	} while (i > 0);
-	i = 8*(sizeof(long) - f->size);
-	if (i) {
-		x <<= i;
-		x >>= i;
-	}
+	/* Extend the sign bit. */
+	if (SIZEOF_LONG > f->size)
+		x |= -(x & (1L << (8*f->size - 1)));
 	return PyInt_FromLong(x);
 }