Move string table to the end of the __LINKEDIT segment.

This change allows Apple's codesign(1) utility to successfully sign
Go binaries, as long as they don't contain DWARF data (-w flag to
8l/6l).  This is because codesign(1) expects the string table to be
the last part of the file.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5271050