include/qemu/lockable: Use _Generic instead of QEMU_GENERIC

include/qemu/lockable: Use _Generic instead of QEMU_GENERIC

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 <[email protected]>
Tested-by: Alex Bennée <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Message-Id: <20210614233143.1221879[email protected]>