Recently NetBSD starts to enforce this, and refuses to execute
the program if n is larger than the sum of entry sizes.

Before:
$ readelf -n ../bin/go.old
Notes at offset 0x00000bd0 with length 0x00000019:
  Owner         Data size       Description
  NetBSD                0x00000004      NT_VERSION (version)
readelf: Warning: corrupt note found at offset 18 into core notes
readelf: Warning:  type: 0, namesize: 00000000, descsize: 00000000
$ readelf -n ../bin/go
Notes at offset 0x00000bd0 with length 0x00000018:
Owner         Data size       Description
NetBSD                0x00000004      NT_VERSION (version)

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/70710043