nasm: address labels 'rel label' vice 'wrt rip'

nasm does not support `label wrt rip', it requires `rel label'. It is
still fully compatible with yasm.

Provide nasm compatibility. No binary change by this patch with yasm on
{x86_64,i686}-fedora13-linux-gnu. Few longer opcodes with nasm on
{x86_64,i686}-fedora13-linux-gnu have been checked as safe.

Change-Id: I488773a4e930a56e43b0cc72d867ee5291215f50