Revert "Makefile: Rename TARGET_DIRS to TARGET_LIST"

This reverts commit 208ecb3e1acc8d55dab49fdf721a86d513691688. This was
causing problems by making DEF_TARGET_LIST pointless and having to
jump through hoops to build on mingw with a dully enabled config.
This includes a change to fix the per-guest TCG test probe which was
added after 208ecb3 and used TARGET_LIST.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/configure b/configure
index b99ebdd..2a7796e 100755
--- a/configure
+++ b/configure
@@ -6212,7 +6212,7 @@
 echo "VERSION=$qemu_version" >>$config_host_mak
 echo "PKGVERSION=$pkgversion" >>$config_host_mak
 echo "SRC_PATH=$source_path" >> $config_host_mak
-echo "TARGET_LIST=$target_list" >> $config_host_mak
+echo "TARGET_DIRS=$target_list" >> $config_host_mak
 if [ "$docs" = "yes" ] ; then
   echo "BUILD_DOCS=yes" >> $config_host_mak
 fi