[ELF] Add --why-extract= to query why archive members/lazy object files are extracted

Similar to D69607 but for archive member extraction unrelated to GC. This patch adds --why-extract=.

Prior art:

GNU ld -M prints
```
Archive member included to satisfy reference by file (symbol)

a.a(a.o)                      main.o (a)
b.a(b.o)                      (b())
```

-M is mainly for input section/symbol assignment <-> output section mapping
(often huge output) and the information may appear ad-hoc.

Apple ld64
```
__Z1bv forced load of b.a(b.o)
_a forced load of a.a(a.o)
```

It doesn't say the reference file.

Arm's proprietary linker
```
Selecting member vsnprintf.o(c_wfu.l) to define vsnprintf.
...
Loading member vsnprintf.o from c_wfu.l.
              definition:  vsnprintf
              reference :  _printf_a
```

---

--why-extract= gives the user the full data (which is much shorter than GNU ld
-Map). It is easy to track a chain of references to one archive member with a
one-liner, e.g.

```
% ld.lld ...