target/openrisc: Add support for ORFPX64A32

This is hardware support for double-precision floating-point using
pairs of 32-bit registers.  Fix latent bugs in the heretofore unused
helper_itofd and helper_ftoid.  Include the bit for cpu "any".
Change the default cpu for linux-user to "any".

Reviewed-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>