cpu: make x/sys/cpu build on non-linux mips64x
cpu: make x/sys/cpu build on non-linux mips64x

Update golang/go#40995

Change-Id: Icab9865e59350c11da32bf8af2d0830001e41323
Reviewed-on: https://go-review.googlesource.com/c/sys/+/265959
Trust: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/sys,golang/sys,golang/sys