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

Begin using i64 quantities to manipulate double-precision values.
On a 64-bit host this will, for the moment, generate less efficient
code; on a 32-bit host code quality should be largely unchanged.
Code quality for 64-bit will be adjusted with a subsequent patch.
Signed-off-by: NRichard Henderson <rth@twiddle.net>