Disable -Wcast-align when building the rts

Disable -Wcast-align when building the rts
This flag seems buggy on GCC 4.1.2, which is the only GCC version we
have that can bootstrap the SPARC build. We end up with lots of supurious
warnings of the form "cast increases required alignment of target type".
Some legitimate warnings can be fixed by adding an intermediate cast to
(void*), but we get others in rts/sm/GCUtils.c concerning the gct var
that look innocuous to me. We could enable this again once we deprecate
support for registerised builds on this arch.