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.