elfcopy: avoid stripping relocations from static binaries

MFC r339350: elfcopy: delete filter_reloc, it is broken and unnecessary

elfcopy contained logic to filter individual relocations in STRIP_ALL
mode. However, this is not valid; relocations emitted by the linker are
required, unless they apply to an entire section being removed (which is
handled by other logic in elfcopy).

Note that filter_reloc was also buggy: for RELA relocation sections it
operated on uninitialized rel.r_info resulting in invalid operation.

The logic most likely needs to be inverted: instead of removing
relocations because their associated symbols are being removed, we must
keep symbols referenced by relocations. That said, in practice we do
not encounter this code path today: objects being stripped are either
dynamically linked binaries which retain .dynsym, or static binaries
with no relocations.

Just remove filter_reloc. This fixes certain cases including statically
linked binaries containing ifuncs. Stripping binaries with relocations
referencing removed symbols was already broken, and after this change
may still be broken in a different way.

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
Sponsored by: The FreeBSD Foundation