remove the 'weak debug info' notes from the README

Debug symbols work anyway as long you *don't* build the binary in a single step:

o32-clang++ test.cpp -g3           # causes dsymutil invocation

o32-clang++ test.cpp -g3 -c        # causes *no* dsymutil invocation
o32-clang++ -g3 test.o -o test     # causes *no* dsymutil invocation

same with gcc.

Getting line numbers may require you to copy the object files (*.o) onto the system you are debugging on.