[PowerPC] Fully define gdtoa settings on powerpc64.

[bdragon]

Summary

The settings in arith.h were not fully defined on powerpc64 after the gdtoa switchover. Generate the missing settings using arithchk.c, similar to what amd64 did for rS114814.

Test Plan

Run kyua test in (at least) /usr/test/lib/libc/stdio.

[jhibbits]

It looks to me like these are unnecessary. Grep is showing they're unused (except Long, but that's typedef'd automatically already).

lib/libc/powerpc64/arith.h

19 ↗

(On Diff #65519)

This is already present.