[clang] [ARM] Fix test using thumb instead arm arch prefix (PR #166416)

[Simon Tatham via cfe-commits]

statham-arm wrote:

In a quick grep of the clang tests I can see a total of 9 cases where `%clang_cc1` is given a triple of the form `armv6m`, `armv7m` or `armv8m`, compared to over 80 cases of `thumbv[678]m`.

@tomershafir, if you want to fix all 9 of those instead of just one, on the basis that that way the direct cc1 invocations more closely match what the driver would have done, I'd approve it.

https://github.com/llvm/llvm-project/pull/166416