fix(linux): Standardize CC ENV setup instructions

[jmenti]

Cross compile environment is similar across u-boot, TF-A, OPTEE, and Linux builds. Move step by step instructions into GCC_ToolChain RST and reference from other build guides using include directive.

Since the CC ENV instructions export CC path using $CROSS_COMPILE_32 and $CROSS_COMPILE_64, use these in legacy build guides as well.

[github-actions]

New warnings found with rstcheck:

source/linux/Foundational_Components_ATF.rst:95: (SEVERE/4) Problems with "include" directive path:
source/linux/Foundational_Components_OPTEE.rst:37: (SEVERE/4) Problems with "include" directive path:
source/linux/Overview/GCC_ToolChain.rst:74: (INFO/1) Duplicate implicit target name: "linux-devkit".
source/linux/Overview/GCC_ToolChain.rst:285: (INFO/1) Duplicate implicit target name: "k3r5-devkit".
source/linux/Foundational_Components/Kernel/_Users_Guide.rst:56: (SEVERE/4) Problems with "include" directive path:
source/linux/Foundational_Components/U-Boot/BG-Setup-K3.rst:56: (SEVERE/4) Problems with "include" directive path:
source/linux/Foundational_Components/U-Boot/BG-Setup-OMAP.rst:53: (SEVERE/4) Problems with "include" directive path:

[github-actions]

New warnings found with rstcheck:

source/linux/Overview/GCC_ToolChain.rst:74: (INFO/1) Duplicate implicit target name: "linux-devkit".
source/linux/Overview/GCC_ToolChain.rst:285: (INFO/1) Duplicate implicit target name: "k3r5-devkit".
source/linux/Foundational_Components/Kernel/_Users_Guide.rst:56: (SEVERE/4) Problems with "include" directive path:

[github-actions]

New warnings found with rstcheck:

source/linux/Overview/GCC_ToolChain.rst:74: (INFO/1) Duplicate implicit target name: "linux-devkit".
source/linux/Overview/GCC_ToolChain.rst:285: (INFO/1) Duplicate implicit target name: "k3r5-devkit".
source/linux/Foundational_Components/Kernel/_Users_Guide.rst:57: (SEVERE/4) Problems with "include" directive path:

[github-actions]

New warnings found with rstcheck:

source/linux/Overview/GCC_ToolChain.rst:74: (INFO/1) Duplicate implicit target name: "linux-devkit".
source/linux/Overview/GCC_ToolChain.rst:285: (INFO/1) Duplicate implicit target name: "k3r5-devkit".
source/linux/Foundational_Components/Kernel/_Users_Guide.rst:58: (SEVERE/4) Problems with "include" directive path: