[CSKY] Fix the conflict of default fpu features and -mfpu option

The arch or cpu has its default fpu features and versions such as fpuv2_sf/fpuv3_sf.
And there is also -mfpu option to specify and override fpu version and features.
For example, C860 has fpuv3_sf/fpuv3_df feature as default, when
-mfpu=fpv2 is given, fpuv3_sf/fpuv3_df is replaced with fpuv2_sf/fpuv2_df.