tests/tcg: cleanup Makefile inclusions

tests/tcg: cleanup Makefile inclusions

Rename Makefile.probe to Makefile.prereqs and make it actually
define rules for the tests.

Rename Makefile to Makefile.target, since it is not a toplevel
makefile.

Rename Makefile.include to Makefile.qemu and disentangle it
from the QEMU Makefile.target, so that it is invoked recursively
by tests/Makefile.include.  Tests are now placed in
tests/tcg/$(TARGET).

Drop the usage of TARGET_BASE_ARCH, which is ignored by everything except
x86_64 and aarch64.  Fix x86 tests by using -cpu max and, while
at it, standardize on QEMU_OPTS for aarch64 tests too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20190807143523.15917-3-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>