Predefine __LITTLE_ENDIAN__

It's a nice consistency with some other targets to predefine
__LITTLE_ENDIAN__ for little-endian NaCl targets.  We'd like
to be able to use this in upstream-friendly newlib headers.

BUG= none
TEST= manually examined -dM output

R=pdox@google.com

Review URL: https://chromiumcodereview.appspot.com/9447064