linuxPackages.nvidia_x11_legacy340: fix cr4 references

Patch was copied from here:
https://devtalk.nvidia.com/default/topic/813458/linux-4-0-rc1-346-47-build-error-_cr4-functions-fix/

Reference to kernel change, also noted in the forum post:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/arch/x86/include/asm/paravirt.h?id=1e02ce4cccdcb9688386e5b8d2c9fa4660b45389

pkgs/os-specific/linux/nvidia-x11/legacy340.nix

@@ -25,6 +25,8 @@ stdenv.mkDerivation {
 
   builder = ./builder-legacy340.sh;
 
+  patches = [ ./nvidia-340.76-kernel-4.0.patch ];
+
   src =
     if stdenv.system == "i686-linux" then
       fetchurl {

pkgs/os-specific/linux/nvidia-x11/nvidia-340.76-kernel-4.0.patch

@@ -0,0 +1,28 @@
+--- a/kernel/nv-pat.c	2015-07-03 08:39:35.417031728 +0200
++++ b/kernel/nv-pat.c	2015-07-03 08:42:15.631838988 +0200
+@@ -35,8 +35,13 @@
+     unsigned long cr0 = read_cr0();
+     write_cr0(((cr0 & (0xdfffffff)) | 0x40000000));
+     wbinvd();
++#if LINUX_VERSION_CODE < KERNEL_VERSION(3, 20, 0)
+     *cr4 = read_cr4();
+     if (*cr4 & 0x80) write_cr4(*cr4 & ~0x80);
++#else
++    *cr4 = __read_cr4();
++    if (*cr4 & 0x80) __write_cr4(*cr4 & ~0x80);
++#endif
+     __flush_tlb();
+ }
+
+@@ -46,7 +51,11 @@
+     wbinvd();
+     __flush_tlb();
+     write_cr0((cr0 & 0x9fffffff));
++#if LINUX_VERSION_CODE < KERNEL_VERSION(3, 20, 0)
+     if (cr4 & 0x80) write_cr4(cr4);
++#else
++    if (cr4 & 0x80) __write_cr4(cr4);
++#endif
+ }
+
+ static int nv_determine_pat_mode(void)