Create new adapter that passes extra flags through NIX_CFLAGS_COMPILE
and NIX_CFLAGS_LINK that instruct linker to eliminate unused symbols (as
opposed to unused objects).

Disk usage save varies greatly: pkgsStatic.mmh won a lot (9.4Mb -> 8.2Mb),
pkgsStatic.coreutils improvement is around 10Kb out of 9Mb.

This patch is imperfect and does not seem to cause rebuild of musl libc,
so improvement may be bigger. Another problem is that it breaks packages
that seems to do partial linkage, e.g busybox:

GEN     include/applet_tables.h include/NUM_APPLETS.h
HOSTCC  applets/usage_pod
CC      applets/applets.o
LD      applets/built-in.o

/nix/store/f7b0y2q80kgnqjkjmh7cng048wsy6mw9-x86_64-unknown-linux-musl-binutils-2.31.1/bin/x86_6
4-unknown-linux-musl-ld: gc-sections requires either an entry or an undefined symbol
collect2: error: ld returned 1 exit status
make[1]: *** [scripts/Makefile.build:264: applets/built-in.o] Error 1
make: *** [Makefile:372: applets_dir] Error 2

So I am looking for advice where and how to apply these changes.

Motivation for this change

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.