Set default LOCAL_MULTILIB only if LOCAL_MODULE_HOST_ARCH isn't restricted

Otherwise we may end up conflict between LOCAL_MODULE_HOST_ARCH and the
default multilib mode.
Also removed the unneeded variants of LOCAL_MODULE_HOST_ARCH.

Change-Id: I9e5a0144da3cb6310be0ddf098738987e51305de
diff --git a/core/host_executable.mk b/core/host_executable.mk
index 53b5fd1..bf3cde1 100644
--- a/core/host_executable.mk
+++ b/core/host_executable.mk
@@ -2,6 +2,7 @@
 my_prefix := HOST_
 include $(BUILD_SYSTEM)/multilib.mk
 
+ifndef LOCAL_MODULE_HOST_ARCH
 ifndef my_module_multilib
 ifeq ($(HOST_PREFER_32_BIT),true)
 my_module_multilib := 32
@@ -10,6 +11,7 @@
 my_module_multilib := first
 endif
 endif
+endif
 
 ifeq ($(my_module_multilib),both)
 ifeq ($(LOCAL_MODULE_PATH_32)$(LOCAL_MODULE_STEM_32),)