[clang] clang: Fix hipstdpar test relying on default target (PR #111975)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> @arsenm what are you actually trying to fix and what do you expect this to do?

Fix not running tests except on linux. We should have maximum host test coverage, and this test has no reason to depend on the host. All it needs is the explicit target instead of relying on the default 

https://github.com/llvm/llvm-project/pull/111975