SH: use register names instead of numbers

[Alexandre Oliva]

This patch replaces all references to literal register numbers in the
SH-specific configuration files with far more readable macros.  In the
case of the md file, a sed script is created out of #defines in the
new sh-md.h, and this script is used to edit sh.md.

I haven't completed testing of this patch, but I'm getting a lot of
failures with -O3 -fPIC that I believe not to have been introduced by
this patch.  Many mova insns are being modified so as to set other
registers, and then the insn becomes unrecognizable because mova can
only set r0.  I'll investigate.

Anyway, assuming the problem is not introduced by this patch (which
I'm going to verify before installing it), ok to install?