Do not skip the first symbol in the symbol table. Any other indexes
into the symbol table (for example, indexes in relocation entries)
will now refer to the symbol following the one that was intended.

Add an object that contains debug relocations, which debug/dwarf
failed to decode correctly. Extend the relocation tests to cover
this case.

Note that the existing tests passed since the symbol following the
symbol that required relocation is also of type STT_SECTION.

Fixes #4107.

R=golang-dev, mikioh.mikioh, iant, iant
CC=golang-dev
https://golang.org/cl/6848044