Design at http://golang.org/s/go12symtab.

This enables some cleanup of the garbage collector metadata
that will be done in future CLs.

This CL does not move the old symtab and pclntab back into
an unmapped section of the file. That's a bit tricky and will be
done separately.

Fixes #4020.

R=golang-dev, dave, cshapiro, iant, r
CC=golang-dev, nigeltao
https://golang.org/cl/11085043