tcg: Workaround macOS 11.2 mprotect bug

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.

As a short-term fix, ignore failures setting up the guard pages.

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



Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1914849
Message-Id: <20210320165720.1813545-3-richard.henderson@linaro.org>