s/Thearch/thearch/

This reduces the amount of exported global variables,
which in turn could make it easier to refactor them later.

Also updated somewhat vague comment about ld.Thearch.
There is no need for Thearch to be exported as Archinit is
called by ld.Main.

Updates #22095

Change-Id: I266b291f6eac0165f70c51964738206e066cea08
Reviewed-on: https://go-review.googlesource.com/103878
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>