cmd/link: improve XCOFF symbol table

cmd/link: improve XCOFF symbol table

This commit improves symbol table for XCOFF format.
It adds symbol alignment, TLS symbols and move the whole symbol table at
the end of the FILE. As relocations in the future external linking will
need symbols' index, we cannot write the symbol table when it's
generated.

Change-Id: I5dcae85b95e538b65f1a128faf56d4e2aa15baf1
Reviewed-on: https://go-review.googlesource.com/c/163998
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>