plan9: reduce the set of architecture-dependent files on Plan 9

From main repo: https://go-review.googlesource.com/#/c/2167

Change-Id: I2043458a432ccd2aa8b206adc107ac7b215e355f
Reviewed-on: https://go-review.googlesource.com/8720
Reviewed-by: Rob Pike <r@golang.org>
12 files changed