Update the build system doc

This adds a reference to the _{32|64|arm|arm64|...|darwin|linux|windows}
variables, and adds information about how LOCAL_MODULE_HOST_OS works.

It also removes many (but not all) obsolete references.

Bug: 23566667
Change-Id: Ief16a5e245eb65ac00fc23d4fc1b7a409a7a64bd