cmd/link: fix isStmt DWARF info

When CL 147160 introduced function ABIs encoded as symbol versions in
the linker, it became slightly more complicated to look up derived
DWARF symbols. It fixed this by introducing a dwarfFuncSym function to
hide this logic, but missed one derived lookup that was done in the
object reader itself. As a result, we lost the isStmt tables from the
compiler, so every PC was marked as a statement in the DWARF info.

Fix this by moving this derived lookup out of the object reader and
into the DWARF code and calling dwarfFuncSym to get the correctly
versioned symbol.

Should fix the linux-amd64-longtest builder.

Updates #27539.

Change-Id: If40d5ba28bab1918ac4ad18fbb5103666b6d978b
Reviewed-on: https://go-review.googlesource.com/c/149605
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/link/internal/ld/dwarf.go

@@ -1219,11 +1219,14 @@ func writelines(ctxt *Link, unit *compilationUnit, ls *sym.Symbol) {
 
 		pciterinit(ctxt, &pcfile, &s.FuncInfo.Pcfile)
 		pciterinit(ctxt, &pcline, &s.FuncInfo.Pcline)
-		pciterinit(ctxt, &pcstmt, &sym.Pcdata{P: s.FuncInfo.IsStmtSym.P})
 
-		if pcstmt.done != 0 {
+		isStmtSym := dwarfFuncSym(ctxt, s, dwarf.IsStmtPrefix, false)
+		if isStmtSym != nil && len(isStmtSym.P) > 0 {
+			pciterinit(ctxt, &pcstmt, &sym.Pcdata{P: isStmtSym.P})
+		} else {
 			// Assembly files lack a pcstmt section, we assume that every instruction
 			// is a valid statement.
+			pcstmt.done = 1
 			pcstmt.value = 1
 		}
 

src/cmd/link/internal/objfile/objfile.go

@@ -320,8 +320,6 @@ overwrite:
 			pc.InlTree[i].Func = r.readSymIndex()
 		}
 
-		s.FuncInfo.IsStmtSym = r.syms.Lookup(dwarf.IsStmtPrefix+s.Name, int(s.Version))
-
 		if !dupok {
 			if s.Attr.OnList() {
 				log.Fatalf("symbol %s listed multiple times", s.Name)

src/cmd/link/internal/sym/symbol.go

@@ -499,7 +499,6 @@ type FuncInfo struct {
 	Pcline      Pcdata
 	Pcinline    Pcdata
 	Pcdata      []Pcdata
-	IsStmtSym   *Symbol
 	Funcdata    []*Symbol
 	Funcdataoff []int64
 	File        []*Symbol