ppc 64-bit clean-up

[Alexandre Oliva]

A cross-assembler to ppc on a host with 64-bit longs fails to
assemble:

       rlwinm 22,22,28,0xf0000000
 
because the signed int (1 << 31) is sign-extended to long, resulting
in an incorrect bit pattern.  Even worse: if `int's were 16-bits wide,
this would invoke undefined behavior.

This patch fixes it, casting the 1 to long, as done in other nearby
constructs.  I'm checking this in as obviously correct.