tcg: Move in_code_gen_buffer and tests to region.c

Shortly, the full code_gen_buffer will only be visible
to region.c, so move in_code_gen_buffer out-of-line.

Move the debugging versions of tcg_splitwx_to_{rx,rw}
to region.c as well, so that the compiler gets to see
the implementation of in_code_gen_buffer.

This leaves exactly one use of in_code_gen_buffer outside
of region.c, in cpu_restore_state.  Which, being on the
exception path, is not performance critical.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>