Only works with DWARF2-enabled GCC 4.x (not the default MinGW GCC).
Fix fastcall symbol names for COFF assembler output.
Add DWARF2 unwind info to COFF assembler output.
Use COFF assembler mode for MinGW builds.
Always enable the DWARF2 handler if compiled with GCC.
Modify unwinding to always return _ff or _c unwind type.
Generate PE object .pdata/.xdata sections for x64 interpreter.
Can drop r12-r15 saves in Windows/x64 interpreter now.
Make external unwinding the default on x64.
It's mandatory on WIN64 due to the abundance of callee-saved regs.
Allow piecewise internal frame unwinding and optional cleanup.
Store ERRMEM, ERRERR and ERRCPP early and copy down later.
Use FRAME_CP for lj_vm_resume.
Add lj_vm_unwind_*_eh variants as landing pads for external unwinder.
Use fastcall for lj_vm_unwind_*.
Can drop r12/r13 saves in POSIX/x64 interpreter now.
Get number of multiple results from C frame.
Add lj_cont_hook: restores multres and dispatch to static ins.
Can use fastcall for lj_dispatch_ins() now.
