[dev.link] cmd/link: avoid allsyms loop in initarray setup

In the linker's symtab() function, avoid looping over the context's
Syms.Allsyms array to locate the entry symbol when setting up the init
array section; do an explicit ABI0 symbol lookup instead. This is a
minor efficiency tweak / code cleanup.

Fixes #20205.

Change-Id: I2ebc17a3cb2cd63e9f5052bc80f1b0ac72c960e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/209838
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -326,12 +326,11 @@ func textsectionmap(ctxt *Link) uint32 {
 }
 
 func (ctxt *Link) symtab() {
-	switch ctxt.BuildMode {
-	case BuildModeCArchive, BuildModeCShared:
-		for _, s := range ctxt.Syms.Allsym {
-			// Create a new entry in the .init_array section that points to the
-			// library initializer function.
-			if s.Name == *flagEntrySymbol && ctxt.HeadType != objabi.Haix {
+	if ctxt.HeadType != objabi.Haix {
+		switch ctxt.BuildMode {
+		case BuildModeCArchive, BuildModeCShared:
+			s := ctxt.Syms.ROLookup(*flagEntrySymbol, sym.SymVerABI0)
+			if s != nil {
 				addinitarrdata(ctxt, s)
 			}
 		}