Fix PR gdb/23558: Use system's 'getcwd' when cross-compiling GDB

Fix PR gdb/23558: Use system's 'getcwd' when cross-compiling GDB

This is a backport of a gnulib fix for the following bug:

  https://sourceware.org/bugzilla/show_bug.cgi?id=23558

The problem reported there is about the replacement of 'getcwd' when
cross-compiling GDB.  With our current gnulib copy, the mechanism for
deciding whether to use the system's 'getcwd' or gnulib's version is
too simplistic and pessimistic, so when cross-compiling we always end
up using gnulib's version, which has a limitation: it cannot handle
the situation when the parent directory doesn't have read permissions.

The solution is to backport the following gnulib commit:

  commit a96d2e67052c879b1bcc5bc461722beac75fc372
  Author: Bruno Haible <bruno@clisp.org>
  Date:   Thu Aug 23 21:13:19 2018 +0200

      getcwd: Add cross-compilation guesses.

gdb/ChangeLog:
2018-09-10  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/23555
	PR gdb/23558
	* gnulib/import/m4/getcwd-path-max.m4: Add cross-compilation
	guesses.