BUILD_HOST_64bit=1 to build host tools in 64-bit

By default we build tool in 32-bit.  This flag allow individual tool and its
dependencies to be built in 64-bit (eg. Emulator).  Fixes to resolve 64-bit
porting issues (in other git) will be submitted seperately.

Change-Id: I486cf7ddac727d3c374ed890857d497c3a69e598
diff --git a/core/combo/HOST_darwin-x86.mk b/core/combo/HOST_darwin-x86.mk
index 193ec16..198c53f 100644
--- a/core/combo/HOST_darwin-x86.mk
+++ b/core/combo/HOST_darwin-x86.mk
@@ -17,11 +17,17 @@
 # Configuration for Darwin (Mac OS X) on x86.
 # Included by combo/select.mk
 
-# We build everything in 32-bit, because some host tools are
-# 32-bit-only anyway (emulator, acc), and because it gives us
+ifneq ($(strip $(BUILD_HOST_64bit)),)
+# By default we build everything in 32-bit, because it gives us
 # more consistency between the host tools and the target.
+# BUILD_HOST_64bit=1 overrides it for tool like emulator
+# which can benefit from 64-bit host arch.
+HOST_GLOBAL_CFLAGS += -m64
+HOST_GLOBAL_LDFLAGS += -m64
+else
 HOST_GLOBAL_CFLAGS += -m32
 HOST_GLOBAL_LDFLAGS += -m32
+endif # BUILD_HOST_64bit
 
 # Use the Mac OSX SDK 10.5 if the build host is 10.6
 build_mac_version := $(shell sw_vers -productVersion)