target-gcc: fix build

Now that $(STAGING_DIR)/usr/bin is no longer in the PATH, we need to
pass the absolute paths to $(TARGET_CC) when building the target gcc
compiler.

This commit fixes the target gcc build problem reported on the list. I
have successfully been able to build a target gcc for ARM, use it to
compile a hello world application on the target and run this
application.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>