[ELF] - Add support for access to most of synthetic sections from linkerscript.

This is important for cases like:

  .sdata        : {
    *(.got.plt .got)
...
  }

That was not supported before as there was no way to get access to 
synthetic sections from script.

More details on review page.

Differential revision: https://reviews.llvm.org/D27040

llvm-svn: 287913