src/x86/win64.S: Support compiling on non-WIN64 platforms

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.