[docs] Better newclass example code in pirbook

Closes GH #802
[cage] re-add -g to CFLAGS even without --debugging

Closes #1135
Prepare for the 6.10.0 release.
[native_pbc] update fingerprints

to avoid TODO tests.
 perl tools/dev/pbc_header.pl --update-fingerprint t/native_pbc/*.pbc
is part of the release process
[docs] clarify release_manager_guide for native_pbc

missed again
[docs] pdd25_concurrency formatting

And add a sentence about our thread-pool.
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
[pmc] fix bignum.i_substract and i_multiply #1144

properly initialize a new temp mpf. do not the existing bn for it.
[dynpmc] rational cast FLOATVAL to double later

try to keep precision until we call the GMP function, which
can handle the enhanced precision cases better
[cage] make headerizer, fix t/codingstd/pmc_docs.t

for rational formatting
[GH #828] Optimize print_n/i and say_n/i

print_n/nc/i/ic and the say variants go through a full Parrot_io_printf with
3 va_start/va_end scenarios and a custom format string parser,
and through char* -> STRING->char* conversion until the fixed
FLOATVAL_FMT or INTVAL_FMT printf is done.
Just to print a single int or number.

This also lost precision for non-double floatval's - if cast to (double).
Now avoided.