Update GCC to 10.3-2021.10

The previous version of GCC, 10-2020-q4-major, is not the latest and
has issues with CMSE that causes PSA API calls to fail when TF-M is
used.

We've switched the Open IoT SDK to use the latest version 10.3-2021.10,
and for consistency across our projects, we update the version used in
this repo too.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>