arm-toolchain: set CVE_VERSION to fix cve-check warnings

Yocto cve-check currently produces numerous warnings like:
    WARNING: gcc-cross-arm-arm-8.3-r2019.03 do_cve_check: gcc:
    Failed to compare arm-8.3 < 10.0 for CVE-2019-15847
In turn this means that some potential CVEs are not reported.

This occurs because PV has been prefixed with "arm-", to allow for
multiple gcc implementations.

Fix this by setting CVE_VERSION to the non-prefixed version.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jon Mason <jon.mason@arm.com>