MFC r339451: objcopy: restore behaviour required by GCC's build

MFC r339451: objcopy: restore behaviour required by GCC's build

In r339350 filter_reloc() was removed, to fix the case of stripping
statically linked binaries with relocations (which may come from ifunc
use, for example).  As a side effect this changed the behaviour when
stripping object files - the output was broken both before and after
r339350, in different ways.  Unfortunately GCC's build process relies
on the previous behaviour, so:

- Revert r339350, restoring filter_reloc().
- Fix an unitialized variable use (commited as r3638 in ELF Tool Chain).
- Change filter_reloc() to omit relocations referencing removed
  symbols, while retaining relocations with no symbol reference.
- Retain the entire relocation section if it references the dynamic
  symbol table (fix from kaiw in D17596).

PR:		232176
Approved by:	re (gjb, kib)
Sponsored by:	The FreeBSD Foundation