android: Support host multilib builds.

The Android build system now supports multilib 32/64-bit host builds as
well as target builds, and Chromium needs to make use of this to
properly support compiling V8 for host. Introduce a GYP_HOST_VAR_PREFIX
variable that works equivalently to GYP_VAR_PREFIX and use it in all the
places where the first/second host architecture must be selected, and
also introduce GYP_HOST_MULTILIB to enable the top level makefile to
specify whether it wants 32-bit or 64-bit host binaries.

BUG=chromium:358141
Review URL: https://codereview.chromium.org/301373002

Patch from Torne <torne@chromium.org>!


git-svn-id: http://gyp.googlecode.com/svn/trunk@1970 78cadc50-ecff-11dd-a971-7dbc132099af