cmd/compile: add coverage fixup mode

cmd/compile: add coverage fixup mode

Adds a -coveragecfg=<configfile> command line option to the compiler
to help support a cooperative "tool and compiler" mode for coverage
instrumentation. In this mode the cmd/cover tool generates most of the
counter instrumentation via source-to-source rewriting, but the
compiler fixes up the result if passed the "-coveragecfg" option. The
fixups include:

  - reclassifying counter variables (special storage class)
  - marking meta-data variables are read-only
  - adding in an init call to do registation

Updates #51430.

Change-Id: Iead72b85209725ee044542374465f118a3ee72e0
Reviewed-on: https://go-review.googlesource.com/c/go/+/395895
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>