add LLVM toolchain support to tests

Created by: strichmo

• Add an LLVM=1 option to the Common Makefile.
• The setup follows the GNU,PULP,COREV toolchain options.
• Add YAML test support to define switches per toolchain (llvm,gnu,pulp,corev).
• BSP builds are now moved into each test_program compilation to ensure march and compile options of BSP matches individual tests.
• cv_regress supports a --toolchain switch to use selected toolchain