[llvm-objdump] Fix dumping dynamic relative relocations for SHT_REL

Previously printing R_386_RELATIVE relocations would trigger
`error: can't read an entry at 0x40: it goes past the end of the section (0x40)`
I found this while writing a test case for LLD (D100490).
This also includes some minor cleanup in the elf-dynamic-relcos.test
llvm-objdump test based on the newly added test.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D100489