[libomptarget][amdgpu] Add hidden_heap_v1 kernarg metadata

Code object version 5 adds support of hidden_heap_v1 kernarg
metadata field [1]. It is a global address space pointer to an
initialized memory buffer that conforms to the requirements of the
malloc/free device library V1 version implementation.

[1] https://llvm.org/docs/AMDGPUUsage.html#amdgpu-amdhsa-code-object-kernel-argument-metadata-map-table-v5

Reviewed By: carlo.bertolli

Differential Revision: https://reviews.llvm.org/D123527