Applying mimmutable(2) to bss, data and text made the sys/kern/noexec tests fail

Applying mimmutable(2) to bss, data and text made the sys/kern/noexec tests fail
since they try to mutate the permissions of the aforementioned sections which is
no longer possible. Instead, "mimic" the sections by allocating and operating on
memory with the same initial permissions.

With help from deraadt@