Carry the gnulib getcwd backport as a patch

Carry the gnulib getcwd backport as a patch

Commit

  e2fc52e7457 ("Fix PR gdb/23558: Use system's 'getcwd' when cross-compiling GDB")

backported some changes from a future gnulib version to our import.
However, this means that every time someone wants to change our gnulib
import (e.g. add a module), they must make sure not to include that
backported change.  It also means that someone running the
update-gnulib.sh script without changes will get some diffs and wonder
why.

Instead, I suggest we carry that backport as a patch applied by the
update-gnulib.sh script after running the import tool.  It will make it
clear what backport or local modification we have and should make
running update-gnulib.sh give a reproducible result.

There is a hunk in the configure file in this patch, this is because the
commit that backported the getcwd bits didn't include the re-generated
configure.

Note: you'll need this patch as well to get deterministic results:

  Generate aclocal-m4-deps.mk more deterministically and portably.
  https://sourceware.org/ml/gdb-patches/2018-09/msg00643.html

gdb/ChangeLog:

	* patches/0001-Fix-PR-gdb-23558-Use-system-s-getcwd-when-cross-comp.patch:
	New file.
	* update-gnulib.sh: Apply patch.
	* configure: Re-generate.