cmd/compile: mark wrapper functions with DW_AT_trampoline

Change DWARF generation to tag wrapper functions with the
"DW_AT_trampoline attribute". The intent is that debuggers can pick up
on this attr so as to skip through the wrapper to the eventual target.

DWARF standard allows for a couple of different possible variants of
the trampoline attr; this is the simplest variant (all it tells the
debugger is that the function is a wrapper, doesn't include a
reference to the wrapper routine).

This implementation keys off the WRAPPER LSym attribute, which is set
for method wrappers, ABI wrappers, and a selected set of runtime
assembly routines (ex: "runtime.call32").

Change-Id: Ib53e1bc56c02b86ca3ac5e7da1a541ec262726cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/347352


Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jeremy Faller <jeremy@golang.org>