Update gdb4hpc documentation

[jussienko]

Current user documentation suggests to build a code with -g. At least Cray Fortran compiler requires that code is compiled without optimization options at all, or with -O0 in order to add debugging information.

Furthermore, gdb4hpc seems to support properly only DWARF-4 debugging format. As Cray C and C++ compilers produce by default DWARF-5, at least with them applications should be build with -gdwarf-4.