Add a tool to convert CFLAGS into CPU_FLAGS

[mgorny]

It has been suggested that we could have a tool that would take an -march= option and output the corresponding CPU_FLAGS_*. I'm thinking the cleanest approach would be to actually to defer to GCC to expand CFLAGS into a set of -m options, and map them onto CPU_FLAGS_*. However, we first need to check if all of the flags have corresponding -m options.