image-mklibs: ensure sysroot is correctly set when calling gcc

image-mklibs: ensure sysroot is correctly set when calling gcc

[YOCTO #2519]

When getting gcc from sstate, it is possible to get a gcc with a bogus
sysroot configuration, as discussed in [1] or in [YOCTO #2519].

mklibs script will eventually call gcc, so we need to make sure that it
provides gcc with the right sysroot location.

[1] http://lists.openembedded.org/pipermail/openembedded-core/2013-September/084159.html

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>