tcg: Add qemu_ld_st_i32/64

Step two in the transition, adding the new ldst opcodes.  Keep the old
opcodes around until all backends support the new opcodes.

Signed-off-by: Richard Henderson <rth@twiddle.net>
exec: Add both big- and little-endian memory helpers

Step three in the transition: helpers not tied to the target
"default" endianness.  To be used when the guest uses a memory
operation with non-default endianness.

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