[Clang] Allow multiple comma separated arguments to `--offload-arch=`

This patch updates the `--[no-]offload-arch` command line arguments to
allow the user to pass in many architectures in a single argument rather
than specifying it multiple times. This means that the following command
line,
```
clang foo.cu --offload-arch=sm_70 --offload-arch=sm_80
```
can become:
```
clang foo.cu --offload-arch=sm_70,sm_80
```

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D128206