gnu: gcc@9: Fix limits.h include for cross builds.

This allows us to supply gcc-9 as an optional argument to cross-gcc,
successfully constructing gcc-9-based cross-compilers.

* gnu/packages/gcc.scm (gcc-9)[source](patches): Add
  "gcc-9-asan-fix-limits-include.patch".
* gnu/packages/patches/gcc-9-asan-fix-limits-include.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add the patch.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/local.mk

@@ -817,6 +817,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gcc-6-source-date-epoch-2.patch		\
   %D%/packages/patches/gcc-8-cross-environment-variables.patch	\
   %D%/packages/patches/gcc-8-strmov-store-file-names.patch	\
+  %D%/packages/patches/gcc-9-asan-fix-limits-include.patch	\
   %D%/packages/patches/gcc-9-strmov-store-file-names.patch	\
   %D%/packages/patches/gd-CVE-2018-5711.patch			\
   %D%/packages/patches/gd-CVE-2018-1000222.patch		\

gnu/packages/gcc.scm

@@ -531,6 +531,7 @@ It also includes runtime support libraries for these languages.")))
              (base32
               "1817nc2bqdc251k0lpc51cimna7v68xjrnvqzvc50q3ax4s6i9kr"))
             (patches (search-patches "gcc-9-strmov-store-file-names.patch"
+                                     "gcc-9-asan-fix-limits-include.patch"
                                      "gcc-5.0-libvtv-runpath.patch"))))))
 
 ;; Note: When changing the default gcc version, update

gnu/packages/patches/gcc-9-asan-fix-limits-include.patch

@@ -0,0 +1,13 @@
+diff --git a/libsanitizer/asan/asan_linux.cc b/libsanitizer/asan/asan_linux.cc
+index d92d0596b7c..7926536a0c3 100644
+--- a/libsanitizer/asan/asan_linux.cc
++++ b/libsanitizer/asan/asan_linux.cc
+@@ -30,7 +30,7 @@
+ #include <sys/types.h>
+ #include <dlfcn.h>
+ #include <fcntl.h>
+-#include <limits.h>
++#include <linux/limits.h>
+ #include <pthread.h>
+ #include <stdio.h>
+ #include <unistd.h>