The main effect of this is to fix anomalies where certain -mfpu options didn't disable everything that they should causing strange behaviour when combined with -mcpu or -march values that themselves enabled fpu subtarget features, e.g. -mfpu=fpv5-dp-d16 with -march=armv7em previously behaved the same as -mfpu=fpv5-sp-d16 due to fp-only-sp not being disabled.
Invalid -mfpu options now also give an error, which is consistent with the handling of the .fpu directive.