htl: Respect GL(dl_stack_flags) when allocating stacks

Previously, HTL would always allocate non-executable stacks.  This has
never been noticed, since GNU Mach on x86 ignores VM_PROT_EXECUTE and
makes all pages implicitly executable.  Since GNU Mach on AArch64
supports non-executable pages, HTL forgetting to pass VM_PROT_EXECUTE
immediately breaks any code that (unfortunately, still) relies on
executable stacks.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-ID: <20240323173301.151066-7-bugaevc@gmail.com>