[binutils] allow immediates in naclrest{bp,sp}

Originally these commands would have produced an error in nacl64-as:
  naclrestbp $0, %r15
  naclrestsp $0, %r15
Error: suffix or operands invalid for `naclrestbp'

8bit immediates would be extended to 32bit.

BUG=
TEST=

Review URL: http://codereview.chromium.org/5648005