Define ABORT_PROVIDED when building for NaCl

libnacl provides a version of abort that exits via a trap rather than
calling exit(1); this is is more useful for debugging. Define ABORT_PROVIDED
so that we always use the version from libnacl instead of newlib's.

R=mcgrathr@chromium.org, mseaborn@chromium.org
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3946
BUG= https://code.google.com/p/nativeclient/issues/detail?id=3248

Review URL: https://codereview.chromium.org/640083002
1 file changed