Fix 64-bit host platform builds.

When performing a 64-bit host platform build, do not pick the 32-bit
SDL prebuilt libraries. This relies on the fact that BUILD_HOST_64bit
will be set in this case.

NOTE: This requires the X11 development headers/libraries to be
      installed on the machine, since SDL will be rebuilt directly
      from sources.

BUG=14489102

Change-Id: Ia36f73f8477059ea1a0d4c48c42fbe2d60595530
diff --git a/Makefile.common b/Makefile.common
index 69961be..2fa464d 100644
--- a/Makefile.common
+++ b/Makefile.common
@@ -202,9 +202,9 @@
 BUILD_SDL_FROM_SOURCES := true
 
 # On linux-x86, using the prebuilts avoid installing all the X11
-# development packages on our build servers.
-#
-ifeq ($(QEMU_HOST_TAG),linux-x86)
+# development packages on our build servers. Note: When building 64-bit
+# host binaries, don't use 32-bit SDL prebuilts.
+ifeq ($(strip $(QEMU_HOST_TAG)$(BUILD_HOST_64bit)),linux-x86)
     BUILD_SDL_FROM_SOURCES := false
 endif