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.