Replace linked list walk with memset.
This reduces CPU time taken by 'go install -a std' by ~10%.
Before:
real		user		sys
0m23.561s	0m16.625s	0m5.848s
0m23.766s	0m16.624s	0m5.846s
0m23.742s	0m16.621s	0m5.868s
after:
0m22.714s	0m14.858s	0m6.138s
0m22.644s	0m14.875s	0m6.120s
0m22.604s	0m14.854s	0m6.081s

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13084043