[ELF] GotSection increment NumEntries when Target saves GlobalOffsetTable in the .got

When the target saves ElfSym::GlobalOffsetTable in the .got rather than
.got.plt, Target->GotHeaderEntriesNum states the number of extra entries
required in the .got. Rather than having to add Target->GotHeaderEntriesNum to
NumEntries in every function which refers to NumEntries, this patch changes the
initial value of NumEntries in the constructor.

Differential Revision: https://reviews.llvm.org/D44744

llvm-svn: 328559