[llvm-readobj/elf][test] - Add testing for EM_* specific OS/ABI values.

We have no tests for OS/ABI values specific to
EM_TI_C6000, ELFOSABI_AMDGPU_MESA3D and ELFOSABI_ARM machines.

Also, related arrays in the code are not grouped together.
(That is why such testing was missed I guess).

The patch fixes that all.

Differential revision: https://reviews.llvm.org/D86341