Preprocess the assembler files.

Preprocess the assembler files.

  * Rename the `*.s' files to `*.S'.

  * Create a new header `base/asm-common.h' containing useful
    definitions, particularly for dealing with the peculiarities of
    shared library code.

  * Convert the assembler files to use the new macros.

  * Convert the assembler files to use `//' for comments rather than
    `#' (as currently).  This is a bit annoying, but `#' is wanted by
    the preprocessor, and `/* ... */' doesn't work in Emacs's
    `asm-mode'.

The reason for doing all of this is because the C preprocessor will let
me do things like inventing symbolic names for registers, which will be
handy later when I add support for AMD64 processors, because most of the
code will be identical between 32- and 64-bit machines.

This change has the side-effect that the AESNI implementation no longer
uses PIC-ish means to find things when it doesn't need to.