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: I8d53510ac081ae20835b3ab7dfee3d74c520ab4d
(cherry picked from commit 7d332965)