[test] Fix unit test case errors.

This fix includes two changes:
- use proper triple name for arm: arm-unknown-linux-gnueabi
- specify output to be in a temp directory

Test cases fixed:
 - clang/test/Driver/fsplit-machine-functions-with-cuda-nvptx.c
 - clang/test/Driver/fsplit-machine-functions.c
 - clang/test/Driver/fsplit-machine-functions2.c

Reviewed: xur