Fix m68k OS ABI sniffer

Do not override the generic OS ABI sniffer.

Fixes: 3eba3a011a8 ("Various m68k fixes for gdb")
diff --git a/gdb/m68k-tdep.c b/gdb/m68k-tdep.c
index 1b8cc92..375d5e6 100644
--- a/gdb/m68k-tdep.c
+++ b/gdb/m68k-tdep.c
@@ -1350,11 +1350,19 @@ static enum gdb_osabi
 m68k_osabi_sniffer (bfd *abfd)
 {
   unsigned int elfosabi = elf_elfheader (abfd)->e_ident[EI_OSABI];
+  enum gdb_osabi osabi = GDB_OSABI_UNKNOWN;
 
   if (elfosabi == ELFOSABI_NONE)
-    return GDB_OSABI_SVR4;
+    {
+      /* Check note sections.  */
+      for (asection *sect : gdb_bfd_sections (abfd))
+	generic_elf_osabi_sniff_abi_tag_sections (abfd, sect, &osabi);
 
-  return GDB_OSABI_UNKNOWN;
+      if (osabi == GDB_OSABI_UNKNOWN)
+	osabi = GDB_OSABI_SVR4;
+    }
+
+  return osabi;
 }
 
 void _initialize_m68k_tdep ();