cmd, runtime: mark assembly routines in FuncFlags

cmd, runtime: mark assembly routines in FuncFlags

There's no good way to ascertain at runtime whether
a function was implemented in assembly.
The existing workaround doesn't play nicely
with some upcoming linker changes.

This change introduces an explicit marker for routines
implemented in assembly.

This change doesn't use the new bit anywhere,
it only introduces it.

Change-Id: I4051dc0afc15b260724a04b9d18aeeb94911bb29
Reviewed-on: https://go-review.googlesource.com/c/go/+/353671
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>