cmd/link: Preallocate Lsym map

Preallocate ~2MB for Lsym map (size calculation from http://play.golang.org/p/9L7F5naXRr).
Reduces best of 10 link time of cmd/go by ~4%.
On cmd/go max resident size unaffected, on println hello world max resident size grows by 4mb from 18mb->22mb. Performance improves in both cases.

tip:  real  0m1.283s user  0m1.502s sys 0m0.144s
this: real  0m1.341s user  0m1.598s sys 0m0.136s

Change-Id: I4a95e45fe552f1f64f53e868421b9f45a34f8b96
Reviewed-on: https://go-review.googlesource.com/19979
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -58,7 +58,8 @@ var headers = []struct {
 
 func linknew(arch *LinkArch) *Link {
 	ctxt := new(Link)
-	ctxt.Hash = make(map[symVer]*LSym)
+	// Preallocate about 2mb for hash
+	ctxt.Hash = make(map[symVer]*LSym, 100000)
 	ctxt.Arch = arch
 	ctxt.Version = obj.HistVersion
 	ctxt.Goroot = obj.Getgoroot()