Port to x86-64 versions of Windows

Port to x86-64 versions of Windows

  - Microsoft x86-64 calling convention differences compared to the
    the System V ABI: Argument passing registers; shadow space.
  - Inform gcc that we are using the System V ABI for a few functions.
  - Define long, unsigned-long to be 32 bit.  This change just falls
    into place now, since incompatible code had been adjusted earlier.
  - Use VEH, not SEH.
  - No pseudo atomic needed around inline allocation, but tweak alloc().
  - Use the gencgc space alignment that also works on win32 x86.
  - Factor "function end breakpoint" handling out of the sigtrap handler.

Beware known bugs, manifested as hangs during threads.impure.lisp,
happening rather frequently with 64 bit builds and at least much
less frequently (or not at all) with 32 bit binaries on the same
version of Windows, tested on Server 2012.  (All credit for features
goes to Anton, all bugs are my fault.)

Thanks to Anton Kovalenko.