Add libgmp 6.2.0 to third_party

Don't build it yet.  That will come in the next review.

Change-Id: Idf3266558165e5ab45f4a41c98cc8c838c8244d5
diff --git a/third_party/gmp/doc/version.texi b/third_party/gmp/doc/version.texi
new file mode 100644
index 0000000..e62c3db
--- /dev/null
+++ b/third_party/gmp/doc/version.texi
@@ -0,0 +1,4 @@
+@set UPDATED 17 January 2020
+@set UPDATED-MONTH January 2020
+@set EDITION 6.2.0
+@set VERSION 6.2.0