api/go1.13: add debug/elf.Symbol fields added in CL 184099

These will need auditing per #32813 like a few others in go1.13.txt, but
in the meantime they break the API check for beta/RC releases.

Updates #32813
Updates #31912

Change-Id: I3b0501b46324ee6fc0985f84971b99b772c7e4a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/189458


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>