Add support for AMD64 processors and Microsoft Windows.

Add support for AMD64 processors and Microsoft Windows.

  * Slightly modify CPU-feature-probing code in `base/dispatch.c',
    mostly to use wider registers for the stack operations since there
    are no 32-bit `push' instructions.  The feature codes are the same
    for both, so there's no corresponding header-file change.

  * Add appropriate macros to `base/asm-common.h' for dealing with PIC
    on AMD64.  It's refreshingly straightforward.

  * Modify the existing assembler code to support the new environments.
    This is mostly tuning register allocation and prologue/epilogue
    sequences.

  * Use the fancy code on the new platforms.