Fix cross-tool check for readelf

This makes all stages of newlib's configury set $READELF to the
proper cross-tool.  This solves the problem that the configure test
for .init_array support would get the wrong answer on Mac, because
it was trying to use the native readelf and there wasn't any.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=1096
TEST= toolchain trybot

R=sehr@google.com

Review URL: http://codereview.chromium.org/8372081
3 files changed