target-alpha: Remove cpu_unique, cpu_sysval, cpu_usp

Technically, these variables could have been referenced both via
offsets from env and as TCG registers, which would be illegal.
Of course, that could only be done from PALcode, and ours doesn't
do that.

But honestly, these are used infrequently enough that they don't
really need to be TCG registers.  We wind up with exactly the same
code if we follow the letter of the law and issue explicit ld/st.

Signed-off-by: Richard Henderson <rth@twiddle.net>
