tests/tcg: invoke Makefile.target directly from QEMU's makefile

Build the "docker.py cc" invocation directly in tests/tcg/configure.sh, and
remove the Makefile.qemu wrapper around Makefile.target.  The config-*.mak
files now include the actual variables used when building the tests, rather
than the CROSS_* variables that Makefile.qemu used to "translate".

This is a first step towards generalizing the cross-compilation infrastructure
so that it can be used for firmware as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220401141326.1244422-15-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-18-alex.bennee@linaro.org>