python extended prompt: Use os.getcwd() instead of os.getcwdu()

It seems like using os.getcwdu() here is wrong both for Python 2 and Python 3.

For Python 2, this returns a 'unicode' object, which tries to get concatenated
to a 'str' object in substitute_prompt. The implicit conversion works when the
unicode string contains no accent. When it does contain an accent though,
displaying the prompt results in the following error:

(gdb) set extended-prompt \w
...
  File "/home/simark/build/binutils-gdb-python2/gdb/data-directory/python/gdb/prompt.py", line 138, in substitute_prompt
    result += str(cmd(arg))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe9' in position 49: ordinal not in range(128)

When using os.getcwd() instead, it works correctly. I suppose that Python does
the necessary decoding internally.

For Python 3, this method simply does not exist. It works fine with os.getcwd().

gdb/ChangeLog:

	* python/lib/gdb/prompt.py (_prompt_pwd): Use os.getcwd() instead of
	os.getcwdu().

gdb/python/lib/gdb/prompt.py

@@ -21,7 +21,7 @@ import os
 
 def _prompt_pwd(ignore):
     "The current working directory."
-    return os.getcwdu()
+    return os.getcwd()
 
 def _prompt_object_attr(func, what, attr, nattr):
     """Internal worker for fetching GDB attributes."""