Support genrules with multiple tools

To allow genrules with more than one tool, rename the tool property to
tools and make it an array, replace $tool with $(location <label>),
and use $() for other variables for consistency.

Bug: 31948427
Test: compare build.ninja
Change-Id: I3d714f70a2af0dc60faeee10e09b6ed166601f1d
(cherry picked from commit b785f5b8)