execute: use prctl(PR_SET_MDWE) for MemoryDenyWriteExecute=yes

execute: use prctl(PR_SET_MDWE) for MemoryDenyWriteExecute=yes

On some ARM platforms, the dynamic linker could use PROT_BTI memory protection
flag with `mprotect(..., PROT_BTI | PROT_EXEC)` to enable additional memory
protection for executable pages. But `MemoryDenyWriteExecute=yes` blocks this
with seccomp filter denying all `mprotect(..., x | PROT_EXEC)`.

Newly preferred method is to use prctl(PR_SET_MDWE) on supported kernels. Then
in-kernel implementation can allow PROT_BTI as necessary, without weakening
MDWE. In-kernel version may also be extended to more sophisticated protections
in the future.