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/4.9/default.nix

@@ -121,6 +121,7 @@ let version = "4.9.4";
         "--enable-sjlj-exceptions"
         "--enable-threads=win32"
         "--disable-win32-registry"
+        "--disable-libmpx" # requires libc
       ] else if crossStageStatic then [
         "--disable-libssp"
         "--disable-nls"
@@ -129,8 +130,9 @@ let version = "4.9.4";
         "--disable-libgomp"
         "--disable-libquadmath"
         "--disable-shared"
-        "--disable-libatomic"  # libatomic requires libc
-        "--disable-decimal-float" # libdecnumber requires libc
+        "--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")