Add 64-bit windows cross-compiles

This is mostly the same as the existing 2ND_HOST / HOST_CROSS support.

The interesting thing I did here was make x86 the 'first' architecture,
and x86_64 the second. This way LOCAL_MULTILIB := first defaults to
32-bit windows modules.

  windows-x86/bin   <- defaults to 32-bit executables
  windows-x86/lib   <- 32-bit libraries, like before
  windows-x86/lib64 <- 64-bit libraries
  windows-x86/obj   <- 32-bit intermediates
  windows-x86/obj64 <- 64-bit intermediates

Then modules are registered with the names:

  host_cross_liblog    <- 32-bit, like before
  host_cross_liblog_64 <- 64-bit

Bug: 26957718
Change-Id: I9f119411acb43e973ec1e6bca3c1dc291c91556c
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 49e56c4..b64b53b 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -297,6 +297,9 @@
 endif
 ifdef HOST_CROSS_OS
 LOCAL_SRC_FILES_$(HOST_CROSS_OS)_$(HOST_CROSS_ARCH):=
+ifdef HOST_CROSS_2ND_ARCH
+LOCAL_SRC_FILES_$(HOST_CROSS_OS)_$(HOST_CROSS_2ND_ARCH):=
+endif
 endif
 
 LOCAL_SRC_FILES_32:=