cmd/asm: fix handling of nested #if[n]defs

The lexer needs to process all #if[n]defs, even those found when processing is
disabled by a preceding failed conditional, or the first #endif in something
like:

    #ifdef <undefined>
    #ifdef whatever
    #endif
    #endif

terminates the first #ifdef and the second causes an error. And then the
processing of the inner #ifdefs needs to ignore their argument when they are
disabled by an outer failed condition.

Change-Id: Iba259498f1e16042f5b7580b9c000bb0599733d0
Reviewed-on: https://go-review.googlesource.com/14253


Reviewed-by: Rob Pike <r@golang.org>