docs(gcc): update GCC to version 10.3-2021.07

This toolchain provides multiple cross compilers and is publicly
available on developer.arm.com

We build TF-A in CI using:
AArch32 bare-metal target (arm-none-eabi)
AArch64 ELF bare-metal target (aarch64-none-elf)

Change-Id: I673e0dce8eb3ca3a004a43158a948431b032e93a
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
1 file changed