In order to be able to scan thread stacks, the Boehm GC needs to know
about newly created threads. We establish the end of the stack by using
GC_call_with_stack_base (this works properly also with the dual-stack
Itanium architecture) and then GC_register_my_thread() to register a
thrad and GC_unregister_my_thread() to unregister it again.
This patch also includes a modification for the refc and markandsweep
collectors to set the stack bottom for thread stacks correctly even if
an optimizer aggressively inlines and optimizes procedures (this is
already being done for the stack of the main thread).
Finally, we use the {.noconv.} pragma for the Boehm GC, as the Boehm
API uses no specific calling convention.
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