Linux: split baseline and architecture requirements

The current Linux compatibility table is difficult to read because it
mixes together multiple architectures. Make things more clear with a
simply architecture-independent table plus a table for arch-specific
requirements.

Change-Id: I6a6a636c9dc87ca5298d7a0bafee59124197c12e
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/749006
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>