tests/tcg: move some multiarch files and make conditional

tests/tcg: move some multiarch files and make conditional

We had some messy code to filter out stuff we can't build. Lets junk
that and simplify the logic by pushing some stuff into subdirs. In
particular we move:

  float_helpers into libs - not a standalone test
  linux-test into linux - so we only build on Linux hosts

This allows for at least some of the tests to be nominally usable
by *BSD user builds.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20210917162332.3511179-4-alex.bennee@linaro.org>