checkpatch: extend line continuation test

Preprocessor directives and asm statements should be allowed to have a
line continuation.

Signed-off-by: Joe Perches <joe@perches.com>
Tested-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>