Add empty symbols to symtab for skipped symbols
LLDB skips several special purpose symbols during symtab parsing (e.g.:
arm/aarch64 mapping symbols) because we don't use them in later stages.
This CL changes the behavior to add an empty symbol to the symtab for
each skipped symbol to preserve the symbol indexes between the symtab in
the object file and LLDBs representation (it is needed because the
relocations are referencing the symbols by index).