It seems the old trick of undef'ing __i386__ was no longer working under
gcc-4.1, so replace the id386 define (controlled by header files) with
USE_X86_ASM (controlled by Makefile).

The checks for id386 in the assembly files are now gone (if you don't want
them, they shouldn't be compiled) and a few extra whitespace cleanups crept
in.

Signed-off-by: Tyrann <tyrann@disenchant.net>