Revert "Makefile: Rename TARGET_DIRS to TARGET_LIST"

This reverts commit 208ecb3e. 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 208ecb3e and used TARGET_LIST.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>