elf: Add <dl-r_debug.h>

Add <dl-r_debug.h> to get the adddress of the r_debug structure after
relocation and its offset before relocation from the PT_DYNAMIC segment
to support DT_DEBUG, DT_MIPS_RLD_MAP_REL and DT_MIPS_RLD_MAP.

Co-developed-by: Xi Ruoyao <xry111@mengyan1223.wang>