We can't rely on the configuration script for this at the moment (see http://lists.llvm.org/pipermail/llvm-dev/2015-October/091758.html).
Even if the configure step was reliable, disabling the whole test, seems like an overkill to me. Given the transition to CMake, I suggest that we commit this and properly configure the test-suite for cross-compilation with CMake in the future.
I'm not keen to change this test in this way for all Mips targets because of a MUSL specific problem. I don't mind a temporary solution but I'd like it to target MUSL specifically.
FWIW, I tested our out-of-tree buildbots and they are passing the test without any problem. This should be a temporary solution as long as we can't use rununder with the CMake build. With CMake it's really easy to check for the existence of these functions during configuration time.