tcg-mips: Layout executable and code_gen_buffer

Choosing good addresses for them means we can use JAL for helper calls.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-mips: Constrain the code_gen_buffer to be within one 256mb segment

This assures us use of J for exit_tb and goto_tb, and JAL for calling
into the generated bswap helpers.

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