[ELF][RISCV] --wrap=foo: Correctly update st_value(foo)

With --wrap=foo, we may have `d->file != file` for a defined symbol `foo`.
For the object file defining `foo`, its symbol table may not contain
`foo` after `redirectSymbols` changed the `foo` entry to `__wrap_foo` (see D50569).

Therefore, skipping `foo` with the condition `if (!d || d->file != file)` may
cause `__wrap_foo` not to be updated. See `ab.o w.o --wrap=foo` in the new test
(originally reported by D150220).

We could adjust the condition to `if (!d)`, but that would leave many `anchors`
entries if a symbol is referenced by many files. Switch to iterating over
`symtab` instead.

Note: D149735 (actually not NFC) allowed duplicate `anchors` entries and fixed
`a.o bw.o --wrap=foo`.

Reviewed By: jobnoorman

Differential Revision: https://reviews.llvm.org/D151768