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.
DynASM/x86: Add lock prefix.
Add support for multilib distro builds.
Fix compilation on NetBSD/x64.

Thanks to Justin Cormack.
DynASM/x64: Add VREG support to mov64.

Thanks to Peter Cawley.
FFI: Properly unload loaded DLLs.
ARM: Fix register allocation when rematerializing FPRs.
Compatibility fix for old GCC versions.
Fix jit.flush(func|true).
Fix argument checks for math.deg()/math.rad().
Fix compiler warning.