The bug referenced in os-win32.h was fixed in mingw-w64 v6.

According to repology, version 5 used by ubuntu 18, which is
not yet out of support, so provide a meson link test for it.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-27-richard.henderson@linaro.org>