Fix alternate stack for Alpine docker on SELinux (#17936)

Fix alternate stack for Alpine docker on SELinux (#17936)

For some reason, the Alpine docker container running on a SELinux host maps
heap as RWX. When we allocate alternate stack from the heap, we also
change the protection of the first page to PROT_NONE so that it can
serve as a guard page to catch stack overflow. And when we free the
alternate stack, we restore the protection back to PROT_READ |
PROT_WRITE. The restoration fails in Alpine docker container running on
a SELinux host with EPROT failure and the SELinux log reports that an
attempt to change heap to executable was made. So it looks like the
kernel has added the PERM_EXEC to the permissions we have passed to the
mprotect call. There is a code in the mprotect implementation that can
do that, although I don't fully understand the conditions under which it
happens. This is driven by the VM_MAYEXEC flag in the internal VMA block
structure.
To fix that, I've modified the alternate stack allocation to use mmap /
munmap instead of C heap allocation.