Add some GNU specific elf constants.

Change-Id: I6c668463a27a641c9a280ce1937857d622344ff8
diff --git a/libc/include/elf.h b/libc/include/elf.h
index 41680a5..ee53ad1 100644
--- a/libc/include/elf.h
+++ b/libc/include/elf.h
@@ -125,4 +125,8 @@
 /* The kernel uses NT_PRFPREG but glibc also offers NT_FPREGSET */
 #define NT_FPREGSET NT_PRFPREG
 
+#define ELF_NOTE_GNU "GNU"
+
+#define NT_GNU_BUILD_ID 3
+
 #endif /* _ELF_H */