[ARM] Fix the extensions implied by a cpu name

Summary:
When using `clang -mcpu=CPUNAME+FEATURELIST`,
the implied features defined by CPUNAME are
not obtained, as the entire string is passed.
This fixes that by spiting the cpuname
string in the first `+`, if any.

For example, when using
```clang -### --target=arm-arm-none-eabi -march=armv7-a -mcpu=cortex-a8+nocrc```
the intrinsic
```"target-feature" "+dsp"```
implied by `cortex-a8` is missing.

Reviewers: keith.walker.arm, DavidSpickett, carwil

Reviewed By: DavidSpickett

Subscribers: javed.absar, kristof.beyls, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61668

llvm-svn: 360324