runtime, internal/cpu: fix build for ARM GNU/Linux

Was failing with

../../../libgo/go/internal/cpu/cpu.go:138:2: error: reference to undefined name 'doinit'
  138 |  doinit()
      |  ^

Fix it by adding in Go 1.12 internal/cpu/cpu_arm.go, and the code in
runtime that initializes the values.

Fixes https://gcc.gnu.org/PR88927.

Change-Id: Ib376fcbd443acf428fa93d4aa77ec74edbaa71ee
Reviewed-on: https://go-review.googlesource.com/c/158717
Reviewed-by: Cherry Zhang <cherryyz@google.com>
2 files changed