[Clang] Fix failing CI with different test case attribute & host triple (#76863)

As in title,
https://github.com/llvm/llvm-project/commit/a8f43974260ec244d78336d2530f8fc097753580
broke CI due to the calling convention not available on certain targets.
This patch uses a simpler calling convention and enables the test only
when the attribute exists. It's verified that this test crashes the
compiler before a8f43974 so it's the
same effect as the previous test. Disabling the test on platforms that
don't have the calling convention is fine because it's guarding against
a frontend bug.