[libomptarget][AMDGPU][NFC] Split atmi_memcpy for h2d and d2h

The calls to atmi_memcpy presently determine the direction of copy (host to
device or device to host) by storing pointers in a map during malloc and
looking up the pointers during memcpy. As each call site already knows the
direction, this stash+lookup can be eliminated.

This NFC will be followed by a functional one that deletes those map lookups.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D89776

Change-Id: I1d9089bc1e56b3a9a30e334735fa07dee1f84990