GCC: Upgrade to version 9.2-2019.12 of toolchain

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

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

Change-Id: I2360a3ac6705c68dca781b85e9894867df255b3e
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/docs/plat/meson-gxbb.rst b/docs/plat/meson-gxbb.rst
index 2cd8342..f891ecd 100644
--- a/docs/plat/meson-gxbb.rst
+++ b/docs/plat/meson-gxbb.rst
@@ -17,7 +17,7 @@
 
 .. code:: shell
 
-    CROSS_COMPILE=aarch64-linux-gnu- make DEBUG=1 PLAT=gxbb bl31
+    CROSS_COMPILE=aarch64-none-elf- make DEBUG=1 PLAT=gxbb bl31
 
 This port has been tested in a ODROID-C2. After building it, follow the
 instructions in the `U-Boot repository`_, replacing the mentioned **bl31.bin**