tests/tcg: merge configure.sh back into main configure script

tests/tcg/configure.sh has a complicated story.

In the beginning its code ran as part of the creation of config-target.mak
files, and that is where it placed the information on the target compiler.
However, probing for the buildability of TCG tests required multiple
inclusions of config-target.mak in the _main_ Makefile (not in
Makefile.target, which took care of building the QEMU executables in
the pre-Meson era), which polluted the namespace.

Thus, it was moved to a separate directory.  It created small config-*.mak
files in $(BUILD_DIR)/tests/tcg.  Those were also included multiple
times, but at least they were small and manageable; this was also an
important step in disentangling the TCG tests from Makefile.target.

Since then, Meson has allowed the configure script to go on a diet.
A few compilation tests survive (mostly for sanitizers) but these days
it mostly takes care of command line parsing, looking for tools, and
setting up the environment for Meson to do its stuff.

It's time to extend configure with the capability to build for more
than just one target: not just tests, but also firmware.  As a first
step, integrate all the logic to find cross compilers in the configure
script, and move tests/tcg/configure.sh back there (though as a
separate loop, not integrated in the one that generates target
configurations for Meson).

tests/tcg is actually very close to being buildable as a standalone
project, so I actually expect the compiler tests to move back to
tests/tcg, as a "configure" script of sorts which would run at Make
time after the docker images are built.  The GCC tree has a similar idea
of doing only bare-bones tree-wide configuration and leaving the rest
for Make time.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220517092616.1272238-8-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220527153603.887929-19-alex.bennee@linaro.org>