Reland "Add prebuilt_build_tool to allow genrules to use prebuilt tools"

Includes fixes for out dirs at absolute paths, and symlinks to absolute
paths. Also squashes support for exporting make vars, which previously
happened in a later patch (which got moved first, since it's mostly
infrastructure changes).

Original message:

cc_prebuilt_binary doesn't work well for host tools because they'll
often be using their own versions of shared libraries that may not be
compatible with what we build.

So add a module type that allows genrules to use one of these prebuilts
as a tool. Like other prebuilts, we'll use the source module if we have
it, or the prebuilt otherwise.

It supports adding extra dependencies for shared libraries or other data
files that are necessary to run the tool. Any genrules using the tool
will be rerun if any of the dependencies change.

Bug: 128690776
Test: treehugger
Test: build one-true-awk using genrule with bison
Test: with OUT_DIR=$PWD/out
Test: with OUT_DIR pointing to an absolute symlink
Change-Id: I662296cb55503b6a10ec5e5465b138e9edeb71f3