gcc: ensure target gcc headers can be included

gcc: ensure target gcc headers can be included

There are a few headers installed as part of gcc-runtime (omp.h,
ssp/*.h). Being installed from a recipe built for the target
architecture, these are within the target sysroot and not
cross/nativesdk; thus they weren't able to be found by gcc with the
existing search paths. Add support for picking up these headers
under the sysroot supplied on the gcc command line in order to
resolve this.

Thanks to Richard Purdie for giving me a number of pointers during
fixing this issue.

Fixes [YOCTO #7141].

(From oe-core master rev: 5c87bb9ac2b35b3f8cf2b7d3e4507e7013115162)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>