ld/Linux: determine program name in a more reliable manner

What argv[0] holds can be pretty arbitrary. As long as it's used for just
diagnostics, that may be pretty okay to go from, but ld also uses it to
find linker scripts. For that we want to be sure we start from the real
executable name. Which on Linux we can determine from the /proc/self/exe
symbolic link target (provided of course procfs is mounted).

While there constify program_name as well.