target/m68k: add fmod/frem

Using a local m68k floatx80_mod()
[copied from previous:
Written by Andreas Grabher for Previous, NeXT Computer Emulator.]

The quotient byte of the FPSR is updated with
the result of the operation.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180224201802.911-3-laurent@vivier.eu>