runtime: ensure arenaBaseOffset makes it into DWARF (for viewcore)

This constant does not make it into DWARF because it is an ideal
constant larger than maxint (1<<63-1). DWARF has no way to represent
signed values that large. Define a different typed constant that
is unsigned and so can represent this constant properly.

Viewcore needs this constant to interrogate the heap data structures.
In addition, the sign of arenaBaseOffset changed in 1.15, and providing
a new name lets viewcore detect the sign change easily.

Change-Id: I4274a2f6e79ebbf1411e85d64758fac1672fb96b
Reviewed-on: https://go-review.googlesource.com/c/go/+/240198


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>