Replace mkDerivedConstants.c with DeriveConstants.hs

DeriveConstants.hs works in a cross-compilation-friendly way. Rather
than running a C program that prints out the constants, we just compile
a C file which has the constants are encoded in symbol sizes. We then
parse the output of 'nm' to find out what the constants are.

Based on work by Gabor Greif <ggreif@gmail.com>.