Reworked libc/docs/gpu/building.rst to match the style of
getting_started.rst:
* Removed mkdir and cd commands.
* Used -S and -B flags for CMake.
* Used -C flag for Ninja.
* Split commands into smaller blocks with brief explanations.
Use the same terminology as elsewhere in the LLVM libc docs and move
away from the deprecated runtime terms.
* Standard runtimes build -> Bootstrap Build
* Runtimes cross build -> Two-stage Cross-compiler Build
6d2d804a22
