gnu: gcc: Fix libsanitizer cross-compilation to powerpc-linux-gnu.

* gnu/packages/patches/gcc-asan-powerpc-missing-include.patch: New file.
* gnu/packages/gcc.scm (gcc-5)[source](patches): Add it.
* gnu/local.mk (dist_patch_DATA): Add it.

gnu/local.mk

@@ -600,6 +600,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gawk-shell.patch				\
   %D%/packages/patches/gcc-arm-bug-71399.patch			\
   %D%/packages/patches/gcc-arm-link-spec-fix.patch		\
+  %D%/packages/patches/gcc-asan-powerpc-missing-include.patch	\
   %D%/packages/patches/gcc-cross-environment-variables.patch	\
   %D%/packages/patches/gcc-libvtv-runpath.patch			\
   %D%/packages/patches/gcc-strmov-store-file-names.patch	\

gnu/packages/gcc.scm

@@ -382,6 +382,7 @@ Go.  It also includes runtime support libraries for these languages.")
                 "0fihlcy5hnksdxk0sn6bvgnyq8gfrgs8m794b1jxwd1dxinzg3b0"))
               (patches (search-patches "gcc-arm-bug-71399.patch"
                                        "gcc-strmov-store-file-names.patch"
+                                       "gcc-asan-powerpc-missing-include.patch"
                                        "gcc-5.0-libvtv-runpath.patch"
                                        "gcc-5-source-date-epoch-1.patch"
                                        "gcc-5-source-date-epoch-2.patch"))))))

gnu/packages/patches/gcc-asan-powerpc-missing-include.patch

@@ -0,0 +1,20 @@
+Add missing include that triggers a build failure on PowerPC:
+
+  ../../../../gcc-5.4.0/libsanitizer/asan/asan_linux.cc: In function ‘bool __asan::AsanInterceptsSignal(int)’:
+  ../../../../gcc-5.4.0/libsanitizer/asan/asan_linux.cc:222:20: error: ‘SIGSEGV’ was not declared in this scope
+     return signum == SIGSEGV && common_flags()->handle_segv;
+		      ^
+From <https://patchwork.ozlabs.org/patch/725596/>.
+
+diff --git a/libsanitizer/asan/asan_linux.cc b/libsanitizer/asan/asan_linux.cc
+index c504168..59087b9 100644
+--- a/libsanitizer/asan/asan_linux.cc
++++ b/libsanitizer/asan/asan_linux.cc
+@@ -29,6 +29,7 @@
+ #include <dlfcn.h>
+ #include <fcntl.h>
+ #include <pthread.h>
++#include <signal.h>
+ #include <stdio.h>
+ #include <unistd.h>
+ #include <unwind.h>