Don't mess up RelIplt symbols during relocatable processing

Summary:
During upgrading of the FreeBSD source tree with lld 7.0.0, I noticed
that it started complaining about crt1.o having an "index past the
end of the symbol table".

Such a symbol table looks approximately like this, viewed with
readelf -s (note the Ndx field being messed up):

Symbol table '.symtab' contains 4 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 SECTION LOCAL  DEFAULT    1
     2: 00000000     0 NOTYPE  WEAK   HIDDEN  RSV[0xffff] __rel_iplt_end
     3: 00000000     0 NOTYPE  WEAK   HIDDEN  RSV[0xffff] __rel_iplt_start

At first, it seemed that recent ifunc relocation work had caused this:
<https://reviews.freebsd.org/rS339351>, but it turned out that it was
due to incorrect processing of the object files by lld, when using -r
(a.k.a. --relocatable).

Bisecting showed that rL324421 ("Convert a use of Config->Static") was
the commit where this new behavior began.  Simply reverting it solved
the issue, and the __rel_iplt symbols had an index of UND again.

Looking at Rafael's commit message, I think he simply missed the
possibility of --relocatable being in effect, so I have added an
additional check for it.

I also added a simple regression test case.

Reviewers: grimar, ruiu, emaste, espindola

Reviewed By: ruiu

Subscribers: arichardson, krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D53515