[clang] [clang][test] Pass -resource-dir in riscv*-toolchain-extra.c tests (PR #226952)

[Michał Górny via cfe-commits]

mgorny wrote:

I don't really know what the test was supposed to test, so I don't know if it's right or not. However, I don't really see the difference between not testing for the path prefix being there and overriding `-clang-resource-dir`, so it will always be there and nothing is really tested.

https://github.com/llvm/llvm-project/pull/226952