[lld] Allow rel iplt symbols with dynamic symbol table

Emit these symbols as long as we're building in a static configuration,
even if we're emitting a dynamic symbol table. This is consistent with
both bfd and gold.

Ordinarily, the combination of -static and -export-dynamic wouldn't make
much sense. Unfortunately, cmake versions prior to 3.4 forcefully
injected -rdynamic [1], so it seems worthwhile to support.

[1] https://cmake.org/cmake/help/v3.4/policy/CMP0065.html

Differential Revision: https://reviews.llvm.org/D36350

llvm-svn: 310168