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.

Also remove the host bin directory from the genrule path, and the
$srcDir variable, using either of them would have caused dependency
issues.

Bug: 31948427
Test: compare build.ninja
Change-Id: Icf6d3bce2bea00fec1363fd65c0bdf96d09281bf
(cherry picked from commit de6bd86d)