Allow not rebasing in cygwin stdenv.

This changes cygwin stdenv, but I don't think it will hurt much people.

This allows mkDerivation to get "dontRebase=true" to skip the usual cygwin
rebase. This is required, if we are using this stdenv to build DLLs for win32
inside x86_64-cygwin, because /bin/rebase crashes at finding an arch mismatch.
Additionally, we don't need any rebase for libraries built by visual studio and
meant for visual studio (my use case).

I'm using nix in x86_64-cygwin to build libraries with visual studio, both for
x86_64 and x86.

pkgs/stdenv/cygwin/rebase-i686.sh

@@ -1,6 +1,9 @@
 postFixupHooks+=(_cygwinFixAutoImageBase)
 
 _cygwinFixAutoImageBase() {
+    if [ "$dontRebase" == 1 ]; then
+        return
+    fi
     find $out -name "*.dll" | while read DLL; do
         if [ -f /etc/rebasenix.nextbase ]; then
             NEXTBASE="$(</etc/rebasenix.nextbase)"

pkgs/stdenv/cygwin/rebase-x86_64.sh

@@ -1,6 +1,9 @@
 postFixupHooks+=(_cygwinFixAutoImageBase)
 
 _cygwinFixAutoImageBase() {
+    if [ "$dontRebase" == 1 ]; then
+        return
+    fi
     find $out -name "*.dll" | while read DLL; do
         if [ -f /etc/rebasenix.nextbase ]; then
             NEXTBASE="$(</etc/rebasenix.nextbase)"