Arm64: drop .bss override

Arm64: drop .bss override

The comment looks bogus (perhaps simply stale, perhaps wrongly copied
from Arm in the first place), and there are also no other precautions
against subsections being used on ELF with .bss. It also doesn't look
to be a good idea to override the custom handlers that ELF and COFF
have (afaict doing so further broke .previous on ELF).

As to the mapping state update - such also doesn't appear to be done
for other section switching, so its original purpose was at best
questionable as well.