base/asm-common.h (x86), and knock-on: Add macros for full-size regs.

These registers get used a lot as pointers, so it's useful to be able to
refer to them as full-width registers more conveniently than `R_sp(r)'.
Introduce (C preprocessor) macros `AX', ..., for this purpose, and use
them extensively.

(Delete the existing `SI' and `DI' macros from `rijndael-x86ish-aesni.S'
which had the same purpose.)