Use ELFOSABI_NONE instead of ELFOSABI_LINUX.

The doesn't seem to be a difference and ELFOSABI_NONE seems to be far more
common:

* Linux doesn't care when loading and puts ELFOSABI_NONE on core dumps.
* Gold and bfd ld produce files with ELFOSABI_NONE.
* Gold and bfd ld seems to ignore EI_OSABI other than for freebsd.
* Gas puts ELFOSABI_NONE in most .o files.

llvm-svn: 248534