android: Add support for multiarch builds.
android: Add support for multiarch builds.

The android build system supports 32/64 bit multiarch builds; add
support for this in the android gyp backend. This requires that you run
gyp twice to generate one complete set of makefiles for each
architecture, and then include both into your toplevel Android.mk with
code like:

GYP_VAR_PREFIX :=
include GypAndroid.$(TARGET_ARCH).mk
GYP_VAR_PREFIX := $(TARGET_2ND_ARCH_VAR_PREFIX)
include GypAndroid.$(TARGET_2ND_ARCH).mk

Changes made in this CL:

1) Set GYP_VAR_PREFIX to the empty string by default (which indicates
building for the primary architecture), to maintain compatibility with
non-multiarch builds.

2) Set LOCAL_MODULE_TARGET_ARCH to indicate that we should only build
the current module for whichever architecture is indicated by
GYP_VAR_PREFIX. This will take effect for target static and dynamic
libraries, and for executables.

3) Set LOCAL_2ND_ARCH_VAR_PREFIX when including base_rules.mk directly
for "none" type targets, since the processing logic for
LOCAL_MODULE_TARGET_ARCH isn't applied by the build system in this case.

3) Pass GYP_VAR_PREFIX when calling local-intermediates-dir or
intermediates-dir-for, and use it when referring to
TARGET_OUT_INTERMEDIATE_LIBARIES, so that we get the correct
intermediate directories.

4) Rename all host targets to include the current target architecture,
so that they will be built separately once per target. Not all host
targets actually need to be built twice, but we can't easily tell which
do. There aren't many in total, so this shouldn't be too expensive.

BUG=chromium:358141
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/216773007
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google