[Driver,ARM] Make -mfloat-abi=soft turn off MVE.

Since `-mfloat-abi=soft` is taken to mean turning off all uses of the
FP registers, it should turn off the MVE vector instructions as well
as NEON and scalar FP. But it wasn't doing so.

So the options `-march=armv8.1-m.main+mve.fp+fp.dp -mfloat-abi=soft`
would cause the underlying LLVM to //not// support MVE (because it
knows the real target feature relationships and turned off MVE when
the `fpregs` feature was removed), but the clang layer still thought
it //was// supported, and would misleadingly define the feature macro
`__ARM_FEATURE_MVE`.

The ARM driver code already has a long list of feature names to turn
off when `-mfloat-abi=soft` is selected. The fix is to add the missing
entries `mve` and `mve.fp` to that list.

Reviewers: dmgreen

Subscribers: kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69025

llvm-svn: 375001