[Libomptarget] Handle dynamic stack sizes for AMD COV5 (#72606)

Summary:
One of the changes in the AMD code-object version five was that kernels
that use an unknown amount of private stack memory now no longer default
to 16 KBs. Instead it emits a flag that indicates the runtime must
provide a value. This patch checks if we must provide such a stack, and
uses the existing handling of the stack environment variable to
configure it.