runtime: save correct pid for new m's on plan9/amd64

The pid field in the Tos structure is a 32-bit value.
Loading a 64-bit word also brings in the next field
which is used for the profiling clock.

LGTM=0intro, aram
R=rsc, 0intro, aram
CC=golang-codereviews, mischief
https://codereview.appspot.com/139560044