CI: use external-arm-toolchain 10.3

The 11.2 release of the Arm GCC uses Broadwell-onwards instructions, but
our CI (and many other users) have pre-Broadwell hardware.

Until 11.3 is released which fixes this, go back to using 10.3 for our CI.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/external-gccarm.yml

@@ -5,4 +5,5 @@ local_conf_header:
   cc: |
     SKIP_RECIPE[gcc-cross-arm] = "Using external toolchain"
     TCMODE = "external-arm"
+    PREFERRED_VERSION_external-arm-toolchain = "10.3-2021.07"
     EXTERNAL_TOOLCHAIN = "${TOPDIR}/toolchains/${TARGET_ARCH}"