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/gmp.info-2 b/third_party/gmp/doc/gmp.info-2
new file mode 100644
index 0000000..de53785
--- /dev/null
+++ b/third_party/gmp/doc/gmp.info-2
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