[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@360324
91177308-0d34-0410-b5e6-
96231b3b80d8