GN an Ninja platform support

[mmarchini]

List GN and Ninja platform support so we can compare with our existing supported platforms.

[DeeDeeG]

Ninja and GN have pre-built binaries for Linux, macOS, and Windows, but I believe it can compile for at least PowerPC and s390 (and I believe this is done already in NodeJS's CI), per nodejs/node#38299 (comment)

Official info:

• https://gn.googlesource.com/gn#getting-a-binary
• https://ninja-build.org/
  • https://github.com/ninja-build/ninja/releases

For what it's worth, I believe depot_tools has both gn and ninja for Linux, macOS and Windows. Which I believe may already be required for building V8?