Stop supporting a GOT with only one reserved entry in static PIE
and ld.so itself: support for that in dynamic objects was removed
in 2010.

Inline RELOC_GOT() into boot_md.c and clean up the result like
boot.c, snag just the three DT_MIPS_* tags needed, and delete the
error case which is verified at build time.

ok visa@

Simplify the handling of the explicit relocations based on ld.so only
have NONE and REL32_64 relocations w/o symbol.

ok visa@

unifdef HAVE_JMPREL, delete dt_pltrelsz handling (which was only used
in the HAVE_JMPREL case anyway), and reduce #includes to match boot.c

ok visa@

For more archs, ld.so itself only needs/uses the arch's "just add load offset"
'relative' relocation. Take advantage of that to simplify ld.so's self-reloc
code:
* give the exceptional archs (hppa and mips64) copies of the current boot.c
as boot_md.c
* teach the Makefile to use boot_md.c when present
* reduce boot.c down to the minimum necessary to handle just relative reloc
* teach the Makefile to fail if the built ld.so has other types of relocs

ok visa@ kettenis@

Simplify the handling of the explicit relocations based on ld.so only
have NONE and REL32_64 relocations w/o symbol.

ok visa@

unifdef HAVE_JMPREL, delete dt_pltrelsz handling (which was only used
in the HAVE_JMPREL case anyway), and reduce #includes to match boot.c

ok visa@

For more archs, ld.so itself only needs/uses the arch's "just add load offset"
'relative' relocation. Take advantage of that to simplify ld.so's self-reloc
code:
* give the exceptional archs (hppa and mips64) copies of the current boot.c
as boot_md.c
* teach the Makefile to use boot_md.c when present
* reduce boot.c down to the minimum necessary to handle just relative reloc
* teach the Makefile to fail if the built ld.so has other types of relocs

ok visa@ kettenis@