target-sparc: Add accessors for single-precision fpr access.

Load, store, and "create destination".  This version attempts to
change the behaviour of the translator as little as possible.  We
previously used cpu_tmp32 as the temporary destination, and we
continue to use that.  This will eventually allow a change in
representation of the fprs.

Change the name of the cpu_fpr array to make certain that all
instances are converted.
Signed-off-by: NRichard Henderson <rth@twiddle.net>