gcc: Clarify Disabling libmpx in cross stage static

This isn't a MUSL thing, but just needed for cross compilation to x86.
No one had tried this when all cross compilation was to linux + glibc,
hence why no one noticed this until recently.

pkgs/development/compilers/gcc/6/default.nix

@@ -105,6 +105,7 @@ let version = "6.4.0";
         "--enable-sjlj-exceptions"
         "--enable-threads=win32"
         "--disable-win32-registry"
+        "--disable-libmpx" # requires libc
       ] else if crossStageStatic then [
         "--disable-libssp"
         "--disable-nls"
@@ -113,11 +114,9 @@ let version = "6.4.0";
         "--disable-libgomp"
         "--disable-libquadmath"
         "--disable-shared"
-        "--disable-libatomic"  # libatomic requires libc
-        "--disable-decimal-float" # libdecnumber requires libc
-        # maybe only needed on musl, PATH_MAX
-        # https://github.com/richfelker/musl-cross-make/blob/0867cdf300618d1e3e87a0a939fa4427207ad9d7/litecross/Makefile#L62
-        "--disable-libmpx"
+        "--disable-libatomic" # requires libc
+        "--disable-decimal-float" # requires libc
+        "--disable-libmpx" # requires libc
       ] else [
         (if crossDarwin then "--with-sysroot=${getLib libcCross}/share/sysroot"
          else                "--with-headers=${getDev libcCross}/include")