tests/tcg: don't iterate through other arch compilers

tests/tcg: don't iterate through other arch compilers

There should only be one compiler per architecture. Those cases where
the same compiler can deal with a different architecture should be
explicitly set for both cross_cc and docker configurations. Otherwise
you get strangeness like:

  --cross-cc-aarch64=/bin/false

causing the logic to attempt to use a locally available
arm-linux-gnueabihf-gcc rather than forcing the use of the docker
image which is what is implied by the command line option.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210512102051.12134-10-alex.bennee@linaro.org>