runtime: convert g.waitreason from string to uint8

Every time I poke at #14921, the g.waitreason string
pointer writes show up.

They're not particularly important performance-wise,
but it'd be nice to clear the noise away.

And it does open up a few extra bytes in the g struct
for some future use.

Change-Id: I7ffbd52fbc2a286931a2218038fda52ed6473cc9
Reviewed-on: https://go-review.googlesource.com/99078


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>