[AMDGPU] Report minimum scratch size in code object v5 and later by default

This change sets
-amdgpu-assume-{external-call-stack-size | dynamic-stack-object-size}
options to zero by default for code object v5 and later. The runtime is
expected to adjust the scratch size if the amdhsa_uses_dynamic_stack bit
in the kernel descriptor is set.

Differential Revision: https://reviews.llvm.org/D128346