[ELF][Hexagon] Replace R_HEXAGON_GOT with R_GOTPLT

R_GOTPLT is relative to .got.plt since D59594. Since R_HEXAGON_GOT
relocations always have 0 r_addend, they can use R_GOTPLT instead.

Reviewed By: sidneym

Differential Revision: https://reviews.llvm.org/D66274

llvm-svn: 369128