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.
gc: add stats and check to ms2

add a gc_sys helper variable, to use only one indirection
gc: new maybe_gc_mark gc vtable

fixes --gc ms which called the ms2! maybe_mark_and_sweep function from
fixed_allocator: allocate_new_pool_arena.

Closes GH #1143
ChangeLog for smoke-me/gc-gh1136+1143 branch
gc: fix wrong inf docs
test: add big gc stress test, to catch ms2 bug
gc: fix ms2 crash with stress/gc.1

Fixes #1143
gc: fix inf Memory_Block prologs, add -D traces

inf gc did not store string/pmc Memory_Block prologs (see pobj.h),
and Parrot_str_copy changed a Buffer_flag, i.e. the str->encoding
from the prev. string.
waste now more space in inf for these prologs.

All gc's are now pretty stable.
Fixes #1136 mostly, but not 100% yet.
gc: fix inf shim interp