scripts: gcc-wrapper: Route the GCC errors to stderr

The GCC wrapper writes any error message from GCC to stdout
along with the messages from the wrapper itself. This is okay
for most case, but when GCC is used with -print-xxx flags,
the stdout output is supposed to be taken as input to some
other build command, so putting error messages in there is
pretty bad. Fix this by writing error messages to stderr.

Change-Id: I4656033f11ba5212fdcc884cc588f8b9d2c23419
Signed-off-by: Shadab Naseem <snaseem@codeaurora.org>