GN: support compiler_prefix, use it for ccache on bots.

GN: support compiler_prefix, use it for ccache on bots.

This makes it considerably easier to use ccache with the Android NDK.
You can now just set
  compiler_prefix = "ccache"
  ndk = "/path/to/ndk"
and we'll use the NDK clang, wrapped with ccache.

The name compiler_prefix is stolen from / compatible with Chrome.

If you have ccache, you can just always leave compiler_prefix="ccache" enabled.
This should make it an unusual thing for humans to have to change cc or cxx.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2281163002

Review-Url: https://codereview.chromium.org/2281163002