… required (#2869)

An upcoming change in Bazel makes the test toolchain required, which
means a compatible
exec platform amongst toolchains must be found
(bazelbuild/bazel@2780393).

Some analysis tests of `py_test` force the target platform to a specific
platform, but before this change didn't register a compatible exec
platform. This can be fixed by registering the target platform as an
exec platform. Since Python targets currently depend on a C++ toolchain
through Bazel's `launcher` and `launcher_maker` and the default
toolchain can't cross-compile to Linux, the host platform still needs to
be kept at highest priority to ensure that cross-compilation isn't
needed on macOS.

Work towards #2850