Disable a test in TestReturnValue on arm64 linux

as described in pr33042, we cannot reliably retrieve the return value on
arm64 in cases it is returned via x8 pointer. I tried to do this as
surgically as possible and disabled it only on targets I know to be
affected, as the code is still useful, even though it can only work on
best-effort basis.

llvm-svn: 303076