meson.build: fix summary display of test compilers

The recent refactoring of configure.sh dropped a number of variables
we relied on for printing out information. Make it simpler.

Fixes: eebf199c (tests/tcg: invoke Makefile.target directly from QEMU's makefile)
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220527153603.887929-5-alex.bennee@linaro.org>