[build] separate rules for libtommath files
[build] separate rules for libtommath files

That way unnecessary rebuilds of .o files are avoided, and we can parallelize
the build process with `make -j3` etc.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp