tests/tcg/configure.sh: add handling for assembler only builds

tests/tcg/configure.sh: add handling for assembler only builds

Up until this point we only handled local compilers or assumed we had
everything in the container. This falls down when we are building QEMU
inside the container.

This special handling only affects tricore for now but I put it in a
case just in case we add any other "special" targets. Setting
CROSS_CC_GUEST is a bit of a hack just to ensure the test runs as we
gate on a detected compiler even though the Makefile won't actually
use it. It also means we display something sane in the configure
output.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210720232703.10650-27-alex.bennee@linaro.org>