Also add exp/regexp to build (forgot before).

At this point I am very confident in exp/regexp's
behavior.  It should be usable as a drop-in
replacement for regexp now.

Later CLs could introduce a CompilePOSIX
to get at traditional POSIX ``extended regular expressions''
as in egrep and also an re.MatchLongest method to
change the matching mode to leftmost longest
instead of leftmost first.  On the other hand, I expect
very few people to use either.

R=r, r, gustavo
CC=golang-dev
https://golang.org/cl/4990041