src/x86/win64.S: Support compiling on non-WIN64 platforms Non-WIN64 versions of the GNU assembler don't support the .seh_* directives for structured exception handling, so wrap them in a macro that compiles to nothing. Handle the registers used for the non-Windows x86-64 calling convention when on a non-Windows platform. Distinguish between cases that should refer to the native argument registers (defined as arg0, arg1, arg2, and arg3) and cases that should always refer to the Windows argument registers.