target-alpha: Convert to new ldst opcodes

Or, partially.  The fundamental primitives for the port are gen_load_mem
and gen_store_mem, which take a callback to emit the memory operation.
For that, we continue to use the original inline functions that forward
to the new ops, rather than replicate the same thing privately.

That said, all free-standing calls to tcg_gen_qemu_* have been converted.
The 32-bit floating-point references now use _i32 opcodes, eliminating
a truncate or extension.

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