[Libomptarget] Make the plugins ingore undefined exported symbols

Summary:
Recent changes made the default behaviour to error when given an
undefined symbol in a version script. A previous patch fixed this for
`libomptarget` by removing the single undefined symbol. However, the
plguins are expected to only define a subset of the availible functions
so we shouldn't treat it as an error. This patch updates the build flags
to work appropriately.