MIPS: math-emu: <MADDF|MSUBF>.S: Fix accuracy (32-bit case)

MIPS: math-emu: <MADDF|MSUBF>.S: Fix accuracy (32-bit case)

Implement fused multiply-add with correct accuracy.

Fused multiply-add operation has better accuracy than respective
sequential execution of multiply and add operations applied on the
same inputs. This is because accuracy errors accumulate in latter
case.

This patch implements fused multiply-add with the same accuracy
as it is implemented in hardware, using 64-bit intermediate
calculations.

One test case example (raw bits) that this patch fixes:

MADDF.S fd,fs,ft:
  fd = 0x22575225
  fs = ft = 0x3727c5ac

Fixes: e24c3bec3e8e ("MIPS: math-emu: Add support for the MIPS R6 MADDF FPU instruction")
Fixes: 83d43305a1df ("MIPS: math-emu: Add support for the MIPS R6 MSUBF FPU instruction")
Signed-off-by: Douglas Leung <[email protected]>
Signed-off-by: Miodrag Dinic <[email protected]>
Signed-off-by: Goran Ferenc <[email protected]>
Signed-off-by: Aleksandar Markovic <[email protected]>
Cc: Douglas Leung <[email protected]>
Cc: Bo Hu <[email protected]>
Cc: James Hogan <[email protected]>
Cc: Jin Qian <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: Petar Jovanovic <[email protected]>
Cc: Raghu Gandham <[email protected]>
Cc: <[email protected]> # 4.7+
Cc: [email protected]
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/16890/
Signed-off-by: Ralf Baechle <[email protected]>