[clang] [CLANG][NFC] Modify test cases to suit assigned default sysroot path (PR #77075)

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Yunzezhu94 wrote:

> > @MaskRay Wasn't it you that mentioned this?
> 
> Yes. I hoped that we could deprecated `DEFAULT_SYSROOT`: https://reviews.llvm.org/D158218 , but the relative `DEFAULT_SYSROOT` does not have a good replacement.

I think since relative DEFAULT_SYSROOT still works, modify these cases is helpful to cross compile developers. And this commit could be delete when DEFAULT_SYSROOT get deprecated.

https://github.com/llvm/llvm-project/pull/77075