Support host multilib build

This change basically ported our target multilib to the host side.
It supports 2 host build modes: x86 and x86_64 multilib build.
For now you need to set "BUILD_HOST_64bit=true" to switch to x86_64
multilib build. Later we'll default to x86_64 build and have a flag
to force 32-bit only build, which may be needed by SDK build.

In host module definition, like in target ones, you can use the
following
LOCAL variables to set up multilib configuration:
LOCAL_MULTILIB: can be "both", "first", "32" or "64".
It also supports the same set of arch or 32-vs-64 specific LOCAL
variables.
By default, it builds only for the first arch.

To keep path compatibility, in x86_64 build files are still output to
out/host/linux-x86; Both 32-bit and 64-bit executables are in
out/host/linux-86/bin;
In x86_64 build 32-bit shared libraries are installed to
out/host/linux-x86/lib32
and 64-bit shared libraries are installed to out/host/linux-x86/lib;
32-bit object files are output to out/host/linux-x86/obj32 and 64-bit
object files
are output to out/host/linux-x86/obj.

Bug: 13751317
Change-Id: I6044f83b7db369a33e05209e8c588eb6dc83409f
diff --git a/core/combo/select.mk b/core/combo/select.mk
index c1a7cc0..8234a8f 100644
--- a/core/combo/select.mk
+++ b/core/combo/select.mk
@@ -18,15 +18,11 @@
 #
 # Inputs:
 #	combo_target -- prefix for final variables (HOST_ or TARGET_)
-#	combo_2nd_arch_prefix -- it's defined if this is loaded for TARGET_2ND_ARCH.
+#	combo_2nd_arch_prefix -- it's defined if this is loaded for the 2nd arch.
 #
 
 # Build a target string like "linux-arm" or "darwin-x86".
-ifdef combo_2nd_arch_prefix
-combo_os_arch := $($(combo_target)OS)-$(TARGET_2ND_ARCH)
-else
-combo_os_arch := $($(combo_target)OS)-$($(combo_target)ARCH)
-endif
+combo_os_arch := $($(combo_target)OS)-$($(combo_target)$(combo_2nd_arch_prefix)ARCH)
 
 combo_var_prefix := $(combo_2nd_arch_prefix)$(combo_target)
 
@@ -90,7 +86,7 @@
   # If we are cross-compiling Windows binaries on Linux
   # then use the linux ccache binary instead.
   ifeq ($(HOST_OS)-$(BUILD_OS),windows-linux)
-    CCACHE_HOST_TAG := linux-$(BUILD_ARCH)
+    CCACHE_HOST_TAG := linux-$(HOST_PREBUILT_ARCH)
   endif
   ccache := prebuilts/misc/$(CCACHE_HOST_TAG)/ccache/ccache
   # Check that the executable is here.