[llvm-objdump] -d: don't display mapping symbols as labels

Similar to D96617 for llvm-symbolizer.

This patch matches the GNU objdump -d behavior to suppress printing
labels for mapping symbols. Mapping symbol names don't convey much
information.

When --show-all-symbols (not in GNU) is specified, we still print
mapping symbols.

Note: the `for (size_t SI = 0, SE = Symbols.size(); SI != SE;)` loops
needs to iterate all mapping symbols, even if they are not displayed.
We use the new field `IsMappingSymbol` to recognize mapping symbols.
This field also enables simplification after D139131.

ELF/ARM/disassemble-all-mapping-symbols.s is enhanced to add `.space 2`.
If `End = std::min(End, Symbols[SI].Addr);` is not correctly set, we
would print a `.word`.

Reviewed By: jhenderson, jobnoorman, peter.smith

Differential Revision: https://reviews.llvm.org/D156190