[CUDA] Add --ptxas-path= flag.

Summary:
This lets you build with one CUDA installation but use ptxas from
another install.

This is useful e.g. if you want to avoid bugs in an old ptxas without
actually upgrading wholesale to a newer CUDA version.

Reviewers: tra

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D27788

llvm-svn: 289847