cmd/ld: correctly compute note size on NetBSD.

Patch from http://gnats.NetBSD.org/48811.

LGTM=iant
R=golang-codereviews, minux.ma, iant
CC=golang-codereviews, tk
https://golang.org/cl/94670047

src/cmd/ld/elf.c

@@ -388,7 +388,7 @@ elfnetbsdsig(ElfShdr *sh, uint64 startva, uint64 resoff)
 {
 	int n;
 
-	n = ELF_NOTE_NETBSD_NAMESZ + ELF_NOTE_NETBSD_DESCSZ + 1;
+	n = rnd(ELF_NOTE_NETBSD_NAMESZ, 4) + rnd(ELF_NOTE_NETBSD_DESCSZ, 4);
 	return elfnote(sh, startva, resoff, n);
 }