tcg: When allocating for !splitwx, begin with PROT_NONE

tcg: When allocating for !splitwx, begin with PROT_NONE

There's a change in mprotect() behaviour [1] in the latest macOS
on M1 and it's not yet clear if it's going to be fixed by Apple.

In this case, instead of changing permissions of N guard pages,
we change permissions of N rwx regions.  The same number of
syscalls are required either way.

[1] https://gist.github.com/hikalium/75ae822466ee4da13cbbe486498a191f

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