adding COPTS to shrink kernel binary size

Hi,

Currently vax GENERIC kernel is built with the default COPTS="-O2".
On the other hand, several m68k (and other) ports have
"-fno-unwind-tables" and "-fno-reorder-blocks" to shrink binary size.

I've tried these options and "-freorder-blocks-algorithm=simple"
that has been added since gcc 6 or 7 that gcc man page says:

>>   -freorder-blocks-algorithm=algorithm
>>       Use the specified algorithm for basic block reordering.  The
>>       algorithm argument can be simple, which does not increase code size
>>       (except sometimes due to secondary effects like alignment), or stc,
>>       the "software trace cache" algorithm, which tries to put all often
>>       executed code together, minimizing the number of branches executed
>>       by making extra copies of code.

It looks significantly reduce -current GENERIC kernel size:

---
default (COPTS="-O2")
>    text    data     bss     dec     hex filename
> 3482718  101432  185756 3769906  398632 netbsd

makeoptions COPTS="-O2 -fno-unwind-tables"
>    text    data     bss     dec     hex filename
> 3067628  101432  185756 3354816  3330c0 netbsd

makeoptions COPTS="-O2 -freorder-blocks-algorithm=simple"
>    text    data     bss     dec     hex filename
> 3173462  101432  185756 3460650  34ce2a netbsd

makeoptions COPTS="-O2 -fno-unwind-tables -freorder-blocks-algorithm=simple"
>    text    data     bss     dec     hex filename
> 2758370  101432  185756 3045558  2e78b6 netbsd

makeoptions COPTS="-O2 -fno-unwind-tables -fno-reorder-blocks"
>    text    data     bss     dec     hex filename
> 2742510  101432  185756 3029698  2e3ac2 netbsd
---

I think it's reasonable to add -fno-unwind-tables and
-freorder-blocks-algorithm=simple for vax, but
I wonder if how many VAXen users has memory size issue.

I also wonder if we should add at least '-fno-unwind-tables"
to MI src/sys/conf/Makefile.kern.inc because no C++ exceptions
in kernel ops..

Thanks,
---
Izumi Tsutsui