[dev.link] cmd/link, cmd/compile: create content addressable pcdata syms

Switch pcdata over to content addressable symbols. This is the last
step before removing these from pclntab_old.

No meaningful benchmarks changes come from this work.

Change-Id: I3f74f3d6026a278babe437c8010e22992c92bd89
Reviewed-on: https://go-review.googlesource.com/c/go/+/247399
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/obj/link.go

@@ -624,11 +624,12 @@ func (s *LSym) CanBeAnSSASym() {
 }
 
 type Pcln struct {
-	Pcsp        Pcdata
-	Pcfile      Pcdata
-	Pcline      Pcdata
-	Pcinline    Pcdata
-	Pcdata      []Pcdata
+	// Aux symbols for pcln
+	Pcsp        *LSym
+	Pcfile      *LSym
+	Pcline      *LSym
+	Pcinline    *LSym
+	Pcdata      []*LSym
 	Funcdata    []*LSym
 	Funcdataoff []int64
 	UsedFiles   map[goobj.CUFileIndex]struct{} // file indices used while generating pcfile
@@ -650,10 +651,6 @@ type Auto struct {
 	Gotype  *LSym
 }
 
-type Pcdata struct {
-	P []byte
-}
-
 // Link holds the context for writing object code from a compiler
 // to be linker input or for reading that input into the linker.
 type Link struct {