FFI: Preserve intermediate string for const char * conversion.

Otherwise the trailing NUL is lost.
PPC: Add plt suffix for external calls from assembler code.
Update changelog.
RELEASE LuaJIT-2.0.2
Improve ULOAD forwarding for open upvalues.
FFI: Add missing GC steps for callback argument conversions.
Merge branch 'master' into torch

Conflicts:
	src/luaconf.h