make-bootstrap-tools.nix: use a patchelf built with -static-{libgcc,libstdc++}

Our bootstrap-files unpacker has always relied on a lot of unstated
assumptions, one of them being that every library has a DT_NEEDED
for librt.so, so patchelf'ing something into the RUNPATH into
librt.so means that it will be searched for every library load in
all of the bootstrap-files.

Unfortunately that assumption is not true for libgcc.

This causes problems, because patchelf links against libgcc (and
against libstdc++, which links against libgcc).  So we can't use
patchelf on libgcc, because it needs libgcc, so patchelf doesn't
work until libgcc is patchelfed.

The robust solution here is to use static linking for the copy of
patchelf that is shipped with the bootstrap-files.  We don't have to
go all the way to a statically linked libc; just -static-libgcc and
-static-libstdc++ are enough to break the circular dependency.

pkgs/stdenv/linux/make-bootstrap-tools.nix

@@ -2,6 +2,10 @@
 
 let
   libc = pkgs.stdenv.cc.libc;
+  patchelf = pkgs.patchelf.overrideAttrs(previousAttrs: {
+    NIX_CFLAGS_COMPILE = (previousAttrs.NIX_CFLAGS_COMPILE or []) ++ [ "-static-libgcc" "-static-libstdc++" ];
+    NIX_CFLAGS_LINK = (previousAttrs.NIX_CFLAGS_LINK or []) ++ [ "-static-libgcc" "-static-libstdc++" ];
+  });
 in with pkgs; rec {