[llvm-objcopy] Preserve ARM and AArch64 mapping symbols

Mapping symbols are required by ARM/AArch64 ELF ABI. They help to
disassemble files correctly and are also used in linkers. Nonetheless,
for executable files, the symbols can be stripped to better resemble
the behavior of GNU's objcopy.

Differential Revision: https://reviews.llvm.org/D117233