cmd/compile: introduce special ssa Aux type for calls

This is prerequisite to moving call expansion later into SSA,
and probably a good idea anyway.  Passes tests.

This is the first minimal CL that does a 1-for-1 substitution
of *ssa.AuxCall for *obj.LSym.  Next step (next CL) is to make
this change for all calls so that additional information can
be stored in AuxCall.

Change-Id: Ia3a7715648fd9fb1a176850767a726e6f5b959eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/237680


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>