PR24697, R_PPC_EMB_SDA21 cannot be used when making a shared object

PR24697, R_PPC_EMB_SDA21 cannot be used when making a shared object

This removes a restriction on various R_PPC_EMB relocations that has
been present for ppc32 since 1996-04-26 git commit e25a798839.  As far
as I know, only those relocs that would require addressing via r2 for
.sdata2/.sbss2 access are disallowed in shared libraries.

	PR 24697
	* elf32-ppc.c (ppc_elf_check_relocs): Call bad_shared_reloc
	when !bfd_link_executable for R_PPC_EMB_SDA2I16 and
	R_PPC_EMB_SDA2REL.  Don't call bad_shared_reloc for any other
	reloc.