This is both more and less complicated than our expansion
using __builtin_choose_expr and __builtin_types_compatible_p.

The expansion through QEMU_MAKE_LOCKABLE_ doesn't work because
we're not emumerating all of the types within the same _Generic,
which results in errors about unhandled cases.  We must also
handle void* explicitly, so that the NULL constant can be used.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210614233143.1221879-7-richard.henderson@linaro.org>