[PowerPC] Restrict various P10 options to P10 only.

This patch attempts to restrict the following P10 options:
```
-mprefixed
-mpcrel
-mpaired-vector-memops
```
To P10 only. This will prevent the use of these options on P9 and earlier.

The behaviour of this patch looks like the following on pre-P10:
```
$ clang -mcpu=pwr9 -mpaired-vector-memops test.c -o test
error: option '-mpaired-vector-memops' cannot be specified without '-mcpu=pwr10'
$ clang -mcpu=pwr9 -mprefixed test.c -o test
error: option '-mprefixed' cannot be specified without '-mcpu=pwr10'
$ clang -mcpu=pwr9 -mprefixed -mpcrel test.c -o test
error: option '-mpcrel' cannot be specified without '-mcpu=pwr10 -mprefixed'
$ clang -mcpu=pwr9 -mpcrel -mprefixed test.c -o test
error: option '-mpcrel' cannot be specified without '-mcpu=pwr10 -mprefixed'
$ clang -mcpu=pwr9 -mpcrel test.c -o test
error: option '-mpcrel' cannot be specified without '-mcpu=pwr10 -mprefixed'
```

Differential Revision: https://reviews.llvm.org/D109652