runtime: repair slice, string, and channel printing in gdb

runtime: repair slice, string, and channel printing in gdb

"Something" changed the names of types in gdb, causing the
pretty-printer matchers to fail to match.  This tracks that
change.

Updated runtime-gdb_test.go to include a slice and a channel printing test.

(The straightforward printing of a slicevar doesn't work because
of compiler DWARF problems describing the slicevar, not gdb problems).

Change-Id: I21607a955b9c894f11ecf3763aea2a6dd59a3f42
Reviewed-on: https://go-review.googlesource.com/c/go/+/235926
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>