target-alpha: Implement WH64EN

Backward compatible cache insn introduced for EV7.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
target-alpha: Disallow literal operand to 1C.30 to 1C.37

Before 64f45e49 we used to have literal checks for 4 of these 8 opcodes.
Confirmed that real hardware doesn't allow them.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>