Add FMT_* constants for the floating-point format field in opcodes and
tweak a few places to use them.  Add enums for various invocations of
FOP and tweak gen_farith and its lone caller accordingly.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>