need more constant constraints

[David Edelsohn]

	First, before you do any work on Altivec, please coordinate with
Apple's GCC group and with me.  It sounds like you are duplicating work
already being developed by others.

	We probably can get rid of the 'S' and 'T' EXTRA_CONSTRAINTS in
the PowerPC port.  I don't know why Kenner originally defined 'S'.  I
stayed with the precedent for 'T', but I don't think either serve a useful
purpose other than consistency check.

David