cmd/link: combine all string data into one symbol

cmd/link: combine all string data into one symbol

This CL introduces a mergestrings pass after the reachability
analysis to combine all reachable go.string."..." character data
symbols into a single symbol.

Shrinks juju by 1.2mb (1.5%).
Shrinks cmd/go by 0.5% when building without DWARF.
No noticable effect on linker speed.

Change-Id: I2ba3e60bf418f65766bda257f6ca9eea26d895b6
Reviewed-on: https://go-review.googlesource.com/20165
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>