gdb: make use of set_tdesc_osabi overload in features/ files

gdb: make use of set_tdesc_osabi overload in features/ files

There are two versions of the set_tdesc_osabi function in GDB:

  void
  set_tdesc_osabi (struct target_desc *target_desc, const char *name)
  {
    set_tdesc_osabi (target_desc, osabi_from_tdesc_string (name));
  }

  void
  set_tdesc_osabi (struct target_desc *target_desc, enum gdb_osabi osabi)
  {
    target_desc->osabi = osabi;
  }

In the gdb/features/ files we call the second of these functions, like
this:

  set_tdesc_osabi (result.get (), osabi_from_tdesc_string ("GNU/Linux"));

This can be replaced with a call to the first set_tdesc_osabi
function, so lets do that.  I think that this makes the features/ code
slightly simpler and easier to understand.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>