runtime: remove remaining references to TheChar

After mdempsky's recent changes, these are the only references to
"TheChar" left in the Go tree. Without the context, and without
knowing the history, this is confusing.

Also rename sys.TheGoos and sys.TheGoarch to sys.GOOS
and sys.GOARCH.

Also change the heap dump format to include sys.GOARCH
rather than TheChar, which is no longer a concept.

Updates #15169 (changes heapdump format)

Change-Id: I3e99eeeae00ed55d7d01e6ed503d958c6e931dca
Reviewed-on: https://go-review.googlesource.com/21647


Reviewed-by: Matthew Dempsky <mdempsky@google.com>