Provide default variant for x86.

The TARGET_ARCH_VARIANT was not being set for x86.  This caused some
difficulties when x86-atom tried to include variant-specific versions
of files.
diff --git a/core/combo/ b/core/combo/
index f466147..629e0cc 100644
--- a/core/combo/
+++ b/core/combo/
@@ -1,6 +1,11 @@
 # Configuration for Linux on x86.
 # Included by combo/select.make
+# Provide a default variant.
+ifeq ($(strip $(TARGET_ARCH_VARIANT)),)
 # right now we get these from the environment, but we should
 # pick them from the tree somewhere
 $(combo_target)CC := $(CC)