[PowerPC] Fix multiple ntp configuration issues

• powerpc time_t is 64 bit, not 32 bit.

• Add definition for powerpc64le.

With this, powerpc64le ntpd and ntpdate operate correctly instead of
corrupting the clock and exiting.

Tested on powerpc64, powerpc64le, and powerpc.

No feedback from cy@.

I am a bit confused as to how SIZEOF_TIME_T being wrong ever worked on
powerpc, it being big endian and all.

Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D26379