Make "set osabi none" really work (PR 22980)

Make "set osabi none" really work (PR 22980)

I was looking for a way to reproduce easily PR 22979 by doing this:

  (gdb) set architecture i386:x86-64
  (gdb) set osabi none

However, I noticed that even though I did "set osabi none", the gdbarch
gdb created was for Linux:

  (gdb) set debug arch 1
  (gdb) set architecture i386:x86-64
  ...
  (gdb) set osabi none
  gdbarch_find_by_info: info.bfd_arch_info i386:x86-64
  gdbarch_find_by_info: info.byte_order 1 (little)
  gdbarch_find_by_info: info.osabi 4 (GNU/Linux)  <--- Wrong?
  gdbarch_find_by_info: info.abfd 0x0
  gdbarch_find_by_info: info.tdep_info 0x0
  gdbarch_find_by_info: Previous architecture 0x1e6fd30 (i386:x86-64)
  selected
  gdbarch_update_p: Architecture 0x1e6fd30 (i386:x86-64) unchanged

This is because the value GDB_OSABI_UNKNOWN has an unclear role,
sometimes meaning "no osabi" and sometimes "please selected
automatically".  Doing "set osabi none" sets the requested osabi to
GDB_OSABI_UNKNOWN, in which case gdbarch_info_fill overrides it with a
value from the target description, or the built-in default osabi.  This
means that it's impossible to force GDB not to use an osabi with "set
osabi".  Since my GDB's built-in default osabi is Linux, it always falls
back to GDB_OSABI_LINUX.

To fix it, I introduced GDB_OSABI_NONE, which really means "I don't want
any osabi".  GDB_OSABI_UNKNOWN can then be used only for "not set yet,
please auto-detect".  GDB_OSABI_UNINITIALIZED now seems unnecessary
since it overlaps with GDB_OSABI_UNKNOWN, so I think it can be removed
and gdbarch_info::osabi can be initialized to GDB_OSABI_UNKNOWN.

gdb/ChangeLog:

	PR gdb/22980
	* defs.h (enum gdb_osabi): Remove GDB_OSABI_UNINITIALIZED, add
	GDB_OSABI_NONE.
	* arch-utils.c (gdbarch_info_init): Don't set info->osabi.
	* osabi.c (gdb_osabi_names): Add "unknown" entry.

gdb/testsuite/ChangeLog:

	PR gdb/22980
	* gdb.base/osabi.exp: New file.