Keep only ELF_RTYPE_CLASS_{PLT|COPY} bits for prelink

prelink runs ld.so with the environment variable LD_TRACE_PRELINKING
set to dump the relocation type class from _dl_debug_bindings.  prelink
has the following relocation type classes:

 #define RTYPE_CLASS_VALID       8
 #define RTYPE_CLASS_PLT         (8|1)
 #define RTYPE_CLASS_COPY        (8|2)
 #define RTYPE_CLASS_TLS         (8|4)

where ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA has a conflict with
RTYPE_CLASS_TLS.

Since prelink only uses ELF_RTYPE_CLASS_PLT and ELF_RTYPE_CLASS_COPY
bits, we should clear the other bits when the DL_DEBUG_PRELINK bit is
set.

	[BZ #19178]
	* elf/dl-lookup.c (RTYPE_CLASS_VALID): New.
	(RTYPE_CLASS_PLT): Likewise.
	(RTYPE_CLASS_COPY): Likewise.
	(RTYPE_CLASS_TLS): Likewise.
	(_dl_debug_bindings): Use RTYPE_CLASS_TLS and RTYPE_CLASS_VALID
	to set relocation type class for DL_DEBUG_PRELINK.  Keep only
	ELF_RTYPE_CLASS_PLT and ELF_RTYPE_CLASS_COPY bits for
	DL_DEBUG_PRELINK.

(cherry picked from commit f3d18efb)
(cherry picked from commit 387011e0)