Fix rpath for components when cross-compiling.

This came up while bringing up the components build for Android. Trying
to run v8_shell failed: out/Debug/v8_shell: error while loading shared
libraries: libv8.cr.so: cannot open shared object file: No such file or
directory

readelf -d out/Debug/v8_shell

shows the problem:
 0x0000000f (RPATH)                      Library rpath: [$ORIGIN/lib/]

For cross-compiles, host shared libraries are placed in /lib/host

BUG=chromium:158821

Review URL: https://codereview.chromium.org/13992005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1607 78cadc50-ecff-11dd-a971-7dbc132099af