[dev.link] cmd/link: convert inltree syms to anonymous in pclntab

The pclntab phase generates a series of "inltree.*" symbols with
inlining related pcdata; these symbols previously were given names and
enterered into the symbol lookup table, but there is no real reason to
do this, since they never need to be looked up when pcln generation is
done. Switch them over to anonymous symbols.

So as to insure that the later symtab phase picks them up correctly,
assign them a type of SGOFUNC instead of SRODATA, and change symtab to
look for this when assigning symbols to groups.

Change-Id: I38225dbb130ad7aea5d16f79cef3d8d388c61c2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/227845


Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>