std.debug: flush SPARC register windows from a new window

flushw and ta 3 flush all windows *except* the current one. So we need to do
this in a new register window to get all of the ones we care about.
3d3aff0da9Alex Rønne Petersen committed on 10/15/2025, 11:59:17 AMparentf21a78b
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