[MC][ELF] Mark Solaris objects as ELFOSABI_SOLARIS

Prompted by D107747 <https://reviews.llvm.org/D107747>, it seems prudent to
mark objects as `ELFOSABI_SOLARIS` on Solaris.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D107748