binutils-unwrapped: force targetPrefix if cross compiling

The binutils build system checks by itself if it is building a cross
toolchain or not and prepends or omits a targetPrefix accordingly. This
means that we can always pass target via configureTargets.

However the binutils build system and our bintools wrapper disagree over
whether we are building a cross toolchain or not sometimes since cross
compilation can be relatively subtle in nixpkgs. For example every use
of crossOverlays will make nixpkgs build a cross toolchain even though
localSystem == crossSystem. The cross infrastructure is also used to
build native binaries with a different stdenv (musl instead of glibc,
clang instead of gcc). In all of these cases stdenv.hostPlatform.config
== stdenv.targetPlatform.config, causing binutils to not prepend a
target prefix. At the same time stdenv.hostPlatform !=
stdenv.targetPlatform causing the bintools wrapper to expect a target
prefix, thus building an incomplete set of bintools. This is why
currently pkgsCross.gnu64 and pkgsCross.musl64 aren't working.

The solution is quite simple however: If we detect that we are building
a cross toolchain in the binutils-unwrapped expression, we force the
targetPrefix with --programprefix and fulfill the expectations of the
bintools wrapper at the same time.

Tested (on x86_64-linux):

* pkgsCross.musl64.hello
* pkgsCross.aarch64-multiplatform.hello
* pkgs.hello

Still not working is pkgsCross.gnu64, since
x86_64-unknown-linux-gnu-stage-final-gcc gets confused about targets
now, so bootstrapping the stdenv fails. Since this wasn't working
previously anyways, it's proably fine to fix this separately.

pkgs/development/tools/misc/binutils/default.nix

@@ -107,8 +107,7 @@ stdenv.mkDerivation {
 
   hardeningDisable = [ "format" "pie" ];
 
-  # TODO(@Ericson2314): Always pass "--target" and always targetPrefix.
-  configurePlatforms = [ "build" "host" ] ++ lib.optional (stdenv.targetPlatform != stdenv.hostPlatform) "target";
+  configurePlatforms = [ "build" "host" "target" ];
 
   configureFlags =
     (if enableShared then [ "--enable-shared" "--disable-static" ]
@@ -126,7 +125,19 @@ stdenv.mkDerivation {
     # RUNPATH instead of RPATH on binaries.  This is important because
     # RUNPATH can be overriden using LD_LIBRARY_PATH at runtime.
     "--enable-new-dtags"
-  ] ++ lib.optionals gold [ "--enable-gold" "--enable-plugins" ];
+
+    # force target prefix. Some versions of binutils will make it empty
+    # if `--host` and `--target` are too close, even if Nixpkgs thinks
+    # the platforms are different (e.g. because not all the info makes
+    # the `config`). Other versions of binutils will always prefix if
+    # `--target` is passed, even if `--host` and `--target` are the same.
+    # The easiest thing for us to do is not leave it to chance, and force
+    # the program prefix to be what we want it to be.
+    "--program-prefix=${targetPrefix}"
+  ] ++ lib.optionals gold [
+    "--enable-gold"
+    "--enable-plugins"
+  ];
 
   doCheck = false; # fails