[OpenMP] libomptarget: Don't map alignment padding to host

In the case of partially mapped structs, libomptarget sometimes adds
padding to device allocations to ensure they are aligned properly.
However, without this patch, it considers that padding to be mapped to
the host, which can cause presence checks (e.g.,
`omp_target_is_present` or a `present` modifier) to misbehave for
unmapped parts of the struct.  This patch keeps the padding but treats
it as unmapped.  See the new test case for examples.

Reviewed By: grokos, jdoerfert

Differential Revision: https://reviews.llvm.org/D149685