Inline RELOC_* into boot_md.c and simplify the code based on what

Inline RELOC_* into boot_md.c and simplify the code based on what
we can verify at build time.  Track dt_pltgot as an Elf_Addr instead
of an Elf_Addr* to eliminat casts on both setting and using.  Set
RELATIVE_RELOC so the ld.so Makefile can verify that it has just
the relocation types we expect.

Nothing depends on archdep.h pulling in other #includes anymore, so delete
the #includes and hide the RELOC_* functions that are only used by lib/csu
behind "#ifdef RCRT0"

Tested with full build.