tcg-ppc64: Tidy register allocation order

Remove conditionalization from tcg_target_reg_alloc_order, relying on
reserved_regs to prevent register allocation that shouldn't happen.
So R11 is now present in reg_alloc_order for __APPLE__, but also now
reserved.

Sort reg_alloc_order into call-saved, call-clobbered, and parameters.
This reduces the effect of values getting spilled and reloaded before
function calls.

Whether or not it is reserved, R2 (TOC) is always call-clobbered.

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