Switch to FreeBSD's elf_common.h.

In particular, it contains all the riscv64 definitions we need, and lets
us clear up a lot of our existing ELF headers.

The other two BSDs seem to have errors and/or gaps in their riscv64
constants.

Test: treehugger
Change-Id: I92e48ef56c52c271ff6ed341b82169aa91f11d98
diff --git a/libc/NOTICE b/libc/NOTICE
index fa3dd2c..2edb78f 100644
--- a/libc/NOTICE
+++ b/libc/NOTICE
@@ -4561,6 +4561,34 @@
 
 -------------------------------------------------------------------
 
+Copyright (c) 2017, 2018 Dell EMC
+Copyright (c) 2000, 2001, 2008, 2011, David E. O'Brien
+Copyright (c) 1998 John D. Polstra.
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions
+are met:
+1. Redistributions of source code must retain the above copyright
+   notice, this list of conditions and the following disclaimer.
+2. Redistributions in binary form must reproduce the above copyright
+   notice, this list of conditions and the following disclaimer in the
+   documentation and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGE.
+
+-------------------------------------------------------------------
+
 Copyright (c)1999 Citrus Project,
 All rights reserved.