bfd_section_from_shdr: Support SHT_RELR sections

If a.so contains an SHT_RELR section, objcopy a.so will fail with:

    a.so: unknown type [0x13] section `.relr.dyn'

This change allows objcopy to work.

bfd/
    * elf.c (bfd_section_from_shdr): Support SHT_RELR.