config: rename config/auto/cpu/x86_64 to amd64

we use the consistent cpuarch amd64 throughout.
do this for the cpu hint also.
[test] fix t/steps/auto/arch-01.t GH #1142

on darwin we need to get the cpuarch from system_profiler SPHardwareDataType
as the environment lies to us. thus on amd64 darwin config::auto::arch
always sets amd64, regardless when we lie about the perl-derived archname.