tests/tcg/Makefile: update to be called from Makefile.target

This make is now invoked from each individual target make with the
appropriate CC and EXTRA_CFLAGS set for each guest. It then includes
additional Makefile.targets from:

  - tests/tcg/multiarch (always)
  - tests/tcg/$(TARGET_BASE_ARCH) (if available)
  - tests/tcg/$(TARGET_NAME)

The order is important as the later Makefile's may want to suppress
TESTS from its base arch profile. Each included Makefile.target is
responsible for adding TESTS as well as defining any special build
instructions for individual tests.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>