Re: centralize CPU feature detection

> Done. I haven't tried Arm support yet, but now I realize the header
> should be named generically, so it's now "pg_cpu.h". Then it can be
> included everywhere.

That makes sense, and simplifies the usage of the header. (However,
the include guard still refers to the old name)

> I don't know. The instruction family names are conventionally all in
> caps, but this is just our signal that we've populated the array. That
> said, a less generic name would better for grep-ability.

Yes, that could work too. But reserving the lowercase "init" symbol in
a very generic header seems like a bad idea (especially for a use case
that isn't used globally), even if Postgres itself doesn't use the
symbol for anything else. "INIT" at least would be unlikely to
conflict with something else.