Restrict the size of code_gen_buffer to 2GB on ppc64, which
lets us assert that everything is reachable with addis+addi
from tb_ret_addr.  This lets us use a max of 4 insns for goto_tb
instead of 7.

Emit the indirect branch portion of goto_tb up front, which
means we only have to update two insns to update any link.
With a 64-bit store, we can update the link atomically, which
may be required in future.

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