It is called from the library initialization code.
On return from this function, SP must point
to the top of the stack region, r0 must point
to the base of the heap region, and r1 must point
to the limit of the heap region.
A user-defined memory model (that is, __rt_stackheap_init() and __rt_heap_extend())
is allocated 16 bytes of storage from the __user_perproc_libspace area
if wanted. It accesses this storage by calling __rt_stackheap_storage() to
return a pointer to its 16-byte region.
This function is not part of the C library standard, but the ARM® C library supports it
as an extension.
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