Enhance getcwd(3) to handle NULL like glibc.

Bug: http://code.google.com/p/android/issues/detail?id=36085
Change-Id: I960a1b585887eb66176c61d29c5c61c239a4003f
diff --git a/libc/Android.mk b/libc/Android.mk
index c66ec59..364f4ca 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -17,7 +17,6 @@
 	unistd/fstatfs.c \
 	unistd/ftime.c \
 	unistd/ftok.c \
-	unistd/getcwd.c \
 	unistd/getdtablesize.c \
 	unistd/gethostname.c \
 	unistd/getopt_long.c \
@@ -275,6 +274,7 @@
 	bionic/fdprintf.c \
 	bionic/fork.c \
 	bionic/fts.c \
+	bionic/getcwd.cpp \
 	bionic/if_nametoindex.c \
 	bionic/if_indextoname.c \
 	bionic/ioctl.c \