[PowerPC] Fully define gdtoa settings on powerpc64.

The settings in arith.h were not fully defined on powerpc64 after the gdtoa
switchover. Generate them using arithchk.c, similar to what AMD64 did for
r114814.

Technically, none of this is necessary in FreeBSD gdtoa, but since the other
platforms have full definitions, we might as well have full definitions
too.

Approved by: jhibbits (in irc)
Differential Revision: https://reviews.freebsd.org/D22775