[PowerPC64LE] Fix Lua 5.2 runtime issue

Lua 5.2 (but not earlier or later versions) has a powerpc platform detection
check.

This check was not using the preferred define __powerpc__ so fails to
trigger on gcc.

It also assumes that all of powerpc is big-endian.

Check a more universal preprocessor define, and add support for the
upcoming FreeBSD PowerPC64LE.

Submitted by: Daniel Kolesa <daniel@octaforge.org>
Reviewed by: luporl
Approved by: portmgr (blanket: runtime fix)
Differential Revision: https://reviews.freebsd.org/D26234