runtime: make sparc64 sigtramp better match solaris amd64
runtime: make sparc64 sigtramp better match solaris amd64

sigtramp was using spots in the stack to save values below the current
frame due to bad math.  Unfortunately, fixing that didn't resolve the issue
encountered.

As such, this commit makes solaris sparc64 sigtramp more closely match the
amd64 sigtramp to make comparison easier for debugging purposes.

It does not appear to cause any regressions, but also does not make
sigtramp work as expected.

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