Add GotEntrySize/GotPltEntrySize to ELF target.

Patch by H.J Lu.

For x86-64 psABI, the entry size of .got and .got.plt sections is 8
bytes for both LP64 and ILP32.  Add GotEntrySize and GotPltEntrySize
to ELF target instead of using size of ELFT::uint.  Now we can generate
a simple working x32 executable.

Differential Revision: http://reviews.llvm.org/D22288

llvm-svn: 275301