Motivation for this change

In some cases, building gcc generates the binaries and manpages prefixed with their architecture tuples, such as armv6l-unknown-linux-gnueabihf-gcc and armv6l-unknown-linux-gnueabihf-g++.1. One of these cases is building cross compilers like in the following:

let
  pkgs = import <nixpkgs> {
    crossSystem = (import <nixpkgs/lib>).systems.examples.raspberryPi;
  };
in
  pkgs.stdenv.cc.man

Currently, the gcc builder creates a g++.1 -> gcc.1 symlink to save space, since g++.1 and gcc.1 have the same contents, but in such cases where the architecture tuple is included in the manpage name, this creates a broken symlink (which, coincidentally, leads to file collisions when attempting to add a cross-compiler and native gcc to a home-manager profile or similar). In such cases, .../share/man/man1 ends up containing a broken symlink like this:

total 720
-r--r--r-- 25 root root  13604 Dec 31  1969 armv6l-unknown-linux-gnueabihf-cpp.1.gz
-r--r--r-- 28 root root 335497 Dec 31  1969 armv6l-unknown-linux-gnueabihf-g++.1.gz
-r--r--r-- 28 root root 335497 Dec 31  1969 armv6l-unknown-linux-gnueabihf-gcc.1.gz
-r--r--r-- 25 root root  11318 Dec 31  1969 armv6l-unknown-linux-gnueabihf-gcov.1.gz
-r--r--r-- 25 root root   2517 Dec 31  1969 armv6l-unknown-linux-gnueabihf-gcov-dump.1.gz
-r--r--r-- 25 root root   4056 Dec 31  1969 armv6l-unknown-linux-gnueabihf-gcov-tool.1.gz
lrwxrwxrwx  4 root root      5 Dec 31  1969 g++.1 -> gcc.1

This pull request should fix the way the symlink is created to handle the normal situation without any arch tuples prefixing the file, with arch prefixes, and, since I wasn't sure about any other corner cases, it also handles if there are both a prefixed and non-prefixed manpage or if there aren't any manpages.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.