[PowerPC] Rename TLS_TPRELGD to TLS_GDIE

Choose a better name, that reflects why the flag is set (GD to IE
optimisation) rather than what the flag produces (TPREL64 reloc on
a single GOT entry replacing a tls_index pair).

	* elf32-ppc.c (TLS_GDIE): Rename from TLS_TPRELGD throughout file.
	Correct comment.
	* elf64-ppc.c (TLS_GDIE): Likewise.