Inline helpers need to be exported unmangled.

__open_2() is used by the fortify implementation of open(2) in
fcntl.h, and as such needs an unmangled C name. For some reason
(inlining?), this doesn't cause problems at the default optimization
level, but does for -O0.

The rest of these didn't cause build failures, but they look suspect
and probably will, we just haven't caught them yet.

(cherry-pick of 658727e1 with conflicts
in stdio.h and string.h.)

Bug: 17784968
Change-Id: I7391a7a8999ee204eaf6abd14a3d5373ea419d5b