When both extconf.rb of generator and parser are run in one process,
the second `require_relative` does nothing.

Suppress warnings for old style function definition and unused
variable.

Align code for arm and x86_64 in parallel.

See the results of `have_type` and `try_compile` in addition to
`have_header` for NEON as well as x86_64.  The former results were
just ignored, and `HAVE_TYPE_` macros are unused too.

Remove `have_type` calls because the next `try_compile` calls check
those types.

Integrate duplicate code by extracting headers, types and
initialization code.