ninja: Only write ar_host, cc_host, cxx_host, ld_host in multi-toolset builds.

No intended functionality change: They are only used for targets with a toolset
of 'host', which only happens only if generator_supports_multiple_toolsets is
true.

Saves a whopping 4 lines (out of ~148000) in ninja files (but it's within
the first 8 lines of build.ninja, so it's at least somewhat visible 4 lines?)

R=fischman@chromium.org, michaelbai@chromium.org

Review URL: https://codereview.chromium.org/18236003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1665 78cadc50-ecff-11dd-a971-7dbc132099af