runtime: rename _m_ to mp [generated]

runtime: rename _m_ to mp [generated]

_g_, _p_, and _m_ are primarily vestiges of the C version of the
runtime, while today we prefer Go-style variable names (generally gp,
pp, and mp).

This change replaces all remaining uses of _m_ with mp. There are very
few remaining and all replacements are trivial.

[git-generate]
cd src/runtime

rf 'mv canpanic._m_ canpanic.mp'
GOOS=solaris \
  rf 'mv semasleep._m_ semasleep.mp'
GOOS=aix GOARCH=ppc64 \
  rf 'mv semasleep._m_ semasleep.mp'

Change-Id: I83690f7b4d4dc57557963100e9a2560ff343f3e8
Reviewed-on: https://go-review.googlesource.com/c/go/+/307813
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>