[llvm-readobj] - Convert gnu-sections.test to use YAML.

gnu-sections.test currently use relocs.obj.elf-x86_64 and
relocs.obj.elf-i386 precompiled objects as an inputs.

These inputs actually initially were introduced to test the
dump of relocations and have almost nothing common with dumping
sections.

Patch converts the test to use yaml2obj. That allows to remove
relocs.obj.elf-i386 binary.
(relocs.obj.elf-x86_64 is still used by another test and can't be removed atm).

Differential revision: https://reviews.llvm.org/D62659

llvm-svn: 362377