target-sparc: Use explicit writes to cpu_fsr

By arranging for explicit writes to cpu_fsr after floating point
operations, we are able to mark the helpers as not writing to
tcg globals, which means that we don't need to invalidate the
integer register set across said calls.

Tested-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
