linux_core_dumper: add a fallback definition for NT_SIGINFO

When building on an old system with outdated headers, this define
might not be available.  Add a fallback to our existing elf header.

Bug: google-breakpad:790
Change-Id: I4dfe7a5cebd414cca3582a1a9cfc983503d5a779
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/1507073
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/client/linux/minidump_writer/linux_core_dumper.cc

@@ -43,6 +43,7 @@
 #include <asm/reg.h>
 #endif
 
+#include "common/linux/elf_gnu_compat.h"
 #include "common/linux/linux_libc_support.h"
 
 namespace google_breakpad {

src/common/linux/elf_gnu_compat.h

@@ -43,4 +43,9 @@
 #define NT_GNU_BUILD_ID 3
 #endif
 
+// Newer Linux systems offer this.
+#ifndef NT_SIGINFO
+#define NT_SIGINFO 0x53494749
+#endif
+
 #endif  // COMMON_LINUX_ELF_GNU_COMPAT_H_