Named location attribute added to `tgt_offload_entry` shall be used by
runtime calls like `ompx_dump_mapping_tables` to print the information
of variables that are mapped to the device. `ompx_dump_mapping_tables`
was printing the wrong location information and this change fixes it.
A sample execution of example before the change:
```
omptarget device 0 info: OpenMP Host-Device pointer mappings after block at libomptarget:0:0:
omptarget device 0 info: Host Ptr Target Ptr Size (B) DynRefCount HoldRefCount Declaration
omptarget device 0 info: 0x0000000000206df0 0x00007f02cdc00000 20000000 1 0 <program-file-loc> at unknown:18:35
```
The change replaces unknown to the mapped symbol and location to the
declaration location.
1a2a0c0dc9