[openmp] Accept directory for libomptarget-bc-path

The commandline flag to specify a particular openmp devicertl library
currently errors like:
```
fatal error: cannot open file
      './runtimes/runtimes-bins/openmp/libomptarget':
      Is a directory
```
CommonArgs successfully appends the directory to the commandline args then
mlink-builtin-bitcode rejects it.

This patch is a point fix to that. If --libomptarget-amdgcn-bc-path=directory
then append the expected name for the current architecture and go on as before.
This is useful for test runners that don't hardcode the architecture.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D109057