[ELF2] Remove target specific code from GotPltSection.

The content of reserved entries of the .got.plt section is target specific.

In particular, on x86_64 the zero entry holds the address of the .dynamic section,
but on AArch64 the same info is stored in the zero entry of the .got section.

Differential revision: http://reviews.llvm.org/D14703

llvm-svn: 253239