ld/6l/8l: First pass at changes to the linker to support NetBSD binaries.

ld/6l/8l: First pass at changes to the linker to support NetBSD binaries.

This will not currently create valid NetBSD binaries because NetBSD requires
an ELF note section to run, otherwise the kernel will throw ENOEXEC. I was
unable to determine an elegant way to add the section, so I am submitting
what I have.

References:
http://www.netbsd.org/docs/kernel/elf-notes.html
http://mail-index.netbsd.org/netbsd-bugs/2001/08/03/0012.html

R=rsc
CC=golang-dev
https://golang.org/cl/5472049