Deprecating #default platform in favor of the inferred host platform

Created by: mzlee

Enabling should_remap_host_platform switches the #default platform to be the inferred host platform instead (one of linux-86_64, windows-x86_64, macosx-x86_64, or freebsd-x86_64). If the configuration cxx.host_platform is set, then use that instead.

This is also an RFC for what kind of deprecation timeline works for the community. The actual effect can be fairly subtle.

Effect:

• Building on multiple platforms will now see a difference between Linux, Windows, MacOS, and FreeBSD flavors
• Any hard coded #default matches for platform_* will no longer match

See https://github.com/facebook/buck/pull/1562 for the original pull request and discussion