Skip the main thread's manual stack guard on Linux

Linux doesn't allocate the whole stack right away, and the kernel has
its own stack-guard mechanism to fault when growing too close to an
existing mapping.  If we map our own guard, then the kernel starts
enforcing a rather large gap above that, rendering much of the possible
stack space useless.

Instead, we'll just note where we expect rlimit to start faulting, so
our handler can report "stack overflow", and trust that the kernel's own
stack guard will work.

Fixes #43052.

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