[ELF][X86] Rename R_RELAX_TLS_GD_TO_IE_END to R_RELAX_TLS_GD_TO_IE_GOTPLT

Summary:
This relocation type is used by R_386_TLS_GD. Its formula is the same as
R_GOTPLT (e.g R_X86_64_GOT{32,64} R_386_TLS_GOTIE). Rename it to be clearer.

Differential Revision: https://reviews.llvm.org/D60941

llvm-svn: 358868