Fix "llvm-objdump -d -r" to show relocations inline for ELF files

This fixes a regression introduced by r182908, which broke
llvm-objdump's ability to display relocations inline in a disassembly
dump for ELF object files.

That change removed a SectionRelocMap from Object/ELF.h, which we
recreate in llvm-objdump.cpp.

I discovered this regression via an out-of-tree test
(test/NaCl/X86/pnacl-hides-sandbox-x86-64.ll) which used llvm-objdump.

Note that the "Unknown" string in the test output on i386 isn't quite
right, but this appears to be a pre-existing bug.

Differential Revision: http://llvm-reviews.chandlerc.com/D2559