[dev.cc] cmd/6a, etc: fix line number accounting during #ifdef

[dev.cc] cmd/6a, etc: fix line number accounting during #ifdef

Lines beginning with #ifdef, #else, #endif were not incrementing
the line number, resulting in bad line number information for
assembly files with #ifdefs.

Example:

	#ifndef GOARCH_ppc64
	#endif
	#ifdef GOARCH_ppc64le
	#endif

	TEXT ·use(SB),7,$0
		RET

Before this change, the line number recorded for use in 6a -S output
(and in the runtime information in the binary) was 4 too low.

Change-Id: I23e599112ec9919f72e53ac82d9bebbbae3439ed
Reviewed-on: https://go-review.googlesource.com/3783
Reviewed-by: Rob Pike <r@golang.org>