When there are plugins, there may not be a unique copy of runtime
functions like goexit, mcall, etc.  So identifying them by entry
address is problematic.  Instead, keep track of each special function
using a field in the symbol table.  That way, multiple copies of
the same runtime function will be treated identically.

Fixes #24351
Fixes #23133

Change-Id: Iea3232df8a6af68509769d9ca618f530cc0f84fd
Reviewed-on: https://go-review.googlesource.com/100739
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>