PR go/88927
    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.
    
    Reviewed-on: https://go-review.googlesource.com/c/158717

From-SVN: r268131