Remove HAVE_ASM_PPC_REL16 references

In bc0cdc49 the configure check for HAVE_ASM_PPC_REL16 was removed
on the grounds that the minimum binutils supports rel16 relocs.  This
is true, but not all references to HAVE_ASM_PPC_REL16 in the sources
were removed.

	* config.h.in: Remove HAVE_ASM_PPC_REL16.
	* sysdeps/powerpc/powerpc32/tls-macros.h: Remove HAVE_ASM_PPC_REL16
	and false branch of conditional.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/swapcontext-common.S:
	Likewise.