Fix default_libdir() on FreeBSD

Fix defaul_libdir() on FreeBSD.  The current behaviour of using
usr/lib64 if that exists is wrong on FreeBSD.  The default should be to
always use usr/lib, even if usr/lib64 exists as a folder in the file
system.  Fix this by checking if we're running on FreeBSD and then
always return 'lib' in default_libdir().

mesonbuild/mesonlib.py

@@ -655,6 +655,8 @@ def default_libdir():
                 return 'lib/' + archpath
         except Exception:
             pass
+    if is_freebsd():
+        return 'lib'
     if os.path.isdir('/usr/lib64') and not os.path.islink('/usr/lib64'):
         return 'lib64'
     return 'lib'