cmd/compile/internal/ssa: fix TestStmtLines for AIX

This commit adapts TestStmtLines for AIX operating system.

Update #25893

Change-Id: I1c76bbd8fc679a66b65ecfbd1ed7745518064eae
Reviewed-on: https://go-review.googlesource.com/c/144958Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/stmtlines_test.go

 package ssa_test
 
 import (
+	"cmd/internal/xcoff"
 	"debug/dwarf"
 	"debug/elf"
 	"debug/macho"
@@ -25,6 +26,10 @@ func open(path string) (*dwarf.Data, error) {
 		return fh.DWARF()
 	}
 
+	if fh, err := xcoff.Open(path); err == nil {
+		return fh.DWARF()
+	}
+
 	return nil, fmt.Errorf("unrecognized executable format")
 }