Re: always use runtime checks for CRC-32C instructions

I wrote:
> I did a more thorough scrape of the buildfarm results. Of 161 animals
> currently reporting configure output on HEAD, we have

Oh ... take "current" with a grain of salt there, because I just noticed
that I typo'd my search condition so that it collected results from all
systems that reported since 2022-Oct, rather than in the last month as
I'd intended. There are just 137 animals currently reporting.

Of those, I broke down the architectures reporting using slicing-by-8:

# select arch,count(*) from results where crc = 'slicing-by-8' group by 1 order by 1;
arch | count
--------------------+-------
aarch64 | 1
macppc | 1
mips64eb; -mabi=64 | 1
mips64el; -mabi=32 | 1
ppc64 (power7) | 4
ppc64 (power8) | 2
ppc64le | 7
ppc64le (power8) | 1
ppc64le (power9) | 15
riscv64 | 2
s390x (z15) | 14
sparc | 1
(12 rows)

The one machine using slicing-by-8 where there might be a better
alternative is arowana, which is CentOS 7 with a pretty ancient gcc
version. So I reject the idea that slicing-by-8 is an appropriate
baseline for comparisons. There isn't anybody who will see an
improvement over current behavior: in the population of interest,
just about all platforms are using CRC instructions with or without
a runtime check.

regards, tom lane