elf.h: add SHT_RISCV_ATTRIBUTES.

Test: treehugger
Change-Id: I1919ae440f56c46aad0948e4535eb6fc90b10895
diff --git a/libc/include/elf.h b/libc/include/elf.h
index 04a73f7..81a50db 100644
--- a/libc/include/elf.h
+++ b/libc/include/elf.h
@@ -229,6 +229,8 @@
 #undef SHT_NUM
 #define SHT_NUM 20
 
+#define SHT_RISCV_ATTRIBUTES 0x70000003
+
 /*
  * Experimental support for SHT_RELR sections. For details, see proposal
  * at https://groups.google.com/forum/#!topic/generic-abi/bX460iggiKg.