The CL 49090043 renamed Ureg structures to Ureg386,
    UregArm and UregAmd64. This broke build on Plan 9,
    since ureg_x86.h includes /386/include/ureg.h, which
    declares a structure named Ureg instead of Ureg386.

R=golang-codereviews, bradfitz
CC=golang-codereviews, rsc
https://golang.org/cl/49260043