Accept functions with DW_AT_linkage_name present

Accept functions with DW_AT_linkage_name present

Currently GDB is not able to debug (Binary generated with Clang) variables
present in shared/private clause of OpenMP Task construct. Please note that
LLVM debugger LLDB is able to debug.

In case of OpenMP, compilers generate artificial functions which are not
present in actual program. This is done to apply parallelism to block of
code.

For non-artifical functions, DW_AT_name attribute should contains the name
exactly as present in actual program.
(Ref# http://wiki.dwarfstd.org/index.php?title=Best_Practices)
Since artificial functions are not present in actual program they not having
DW_AT_name and having DW_AT_linkage_name instead should be fine.

Currently GDB is invalidating any function not havnig DW_AT_name which is why
it is not able to debug OpenMP (Clang).

It should be fair to fallback to check DW_AT_linkage_name in case DW_AT_name
is absent.