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>

docs/plat/rpi4.rst

@@ -22,7 +22,7 @@ one universal binary (bl31.bin), which can be built with:
 
 .. code:: shell
 
-    CROSS_COMPILE=aarch64-linux-gnu- make PLAT=rpi4 DEBUG=1
+    CROSS_COMPILE=aarch64-none-elf- make PLAT=rpi4 DEBUG=1
 
 Copy the generated build/rpi4/debug/bl31.bin to the SD card, either
 renaming it to ``armstub8.bin`` or adding an entry starting with ``armstub=``,

docs/plat/ti-k3.rst

@@ -27,7 +27,7 @@ TF-A:
 
 .. code:: shell
 
-    make CROSS_COMPILE=aarch64-linux-gnu- PLAT=k3 SPD=opteed all
+    make CROSS_COMPILE=aarch64-none-elf- PLAT=k3 SPD=opteed all
 
 OP-TEE: