Fix #101: Add -ftrapping-math to clang compiler options
Because lisp runs with FP traps enabled, we want the C code to honor that. Hence add -ftrapping-math.
Also cleaned up the options, replacing -march=pentium4 and -mtune=generic with just -mtune=pentiume4. Replace -mfpmath=sse (not sure what that does) with -msse2.
Finally, update CI to do a clang build so we can verify this actually works.