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.

This is a re-roll of CL 99078, which was rolled
back because of failures on s390x.
Those failures were apparently due to an old version of gdb.

Change-Id: Icc2c12f449b2934063fd61e272e06237625ed589
Reviewed-on: https://go-review.googlesource.com/111256


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>