tests/tcg: remove -f from Makefile invocation

Instead of linking tests/tcg/Makefile.target into the build tree, name
the symbolic link "Makefile" and create it in every target subdirectory.
This makes it possible to just invoke "make" in tests/tcg subdirectories.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929114231.583801-21-alex.bennee@linaro.org>