runtime: remove redundant register saves
runtime: remove redundant register saves

Sigtramp uses a register window and even if it did not, the semantics
of signal handlers for sparc guarantee restoration of the registers upon
return, so there's no need to save the %i registers.

This also doesn't fix signal handling, but also doesn't appear to cause any
regressions.

4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go,4ad/go