[Libmoptarget] Enable the shared allocator for AMDGPU

Currently, the AMDGPU plugin did not support the `TARGET_ALLOC_SHARED`
allocation kind. We used the fine-grained memory allocator for the
"host" alloc when this is most likely not what is intended. Fine-grained
memory can be accessed by all agents, so it should be considered shared.
This patch removes the use of fine-grained memory for the host
allocator. A later patch will add support for this via the
`hsa_amd_memory_lock` method.

Reviewed By: kevinsala

Differential Revision: https://reviews.llvm.org/D143771