BaseTools: Switch to downloading the AARCH64 compiler from Arm's site

BaseTools: Switch to downloading the AARCH64 compiler from Arm's site

Linaro no longer do gcc releases - they're done by Arm now.

Update gcc_aarch64_linux_ext_dep.yaml to fetch the latest AARCH64 gcc
release (10.3-2021.07) from their site and fix LinuxGcc5ToolChain.py with the
new GCC_AARCH64_PREFIX.

Signed-off-by: Rebecca Cran <rebecca@bsdio.com>
Acked-by: Leif Lindholm <leif@nuviainc.com>
Acked-by: Liming Gao <gaoliming@byosoft.com.cn>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Pierre Gondois <Pierre.Gondois@arm.com>
Acked-by: Bob Feng <bob.c.feng@intel.com>