[ELF] Do not produce DT_JMPREL and DT_PLTGOT if .rela.plt is empty.

If .rela.plt is mentioned in a linker script, it might be preserved
even if it is empty. In that case, LLD created DT_JMPREL and DT_PLTGOT
dynamic tags. When the tags exist, a dynamic loader writes values into
reserved slots in .got.plt to support lazy symbol resolution.
The problem is that, in fact, the linker has not reserved that space,
and the writing may occur into the memory allocated for something else.

Differential Revision: https://reviews.llvm.org/D63869

llvm-svn: 364639