libc: Add O_PATH support for fgetxattr / fsetxattr

Support O_PATH file descriptors when handling fgetxattr and fsetxattr.
This avoids requiring file read access to pull extended attributes.

This is needed to support O_PATH file descriptors when calling
SELinux's fgetfilecon() call. In particular, this allows the querying
and setting of SELinux file context by using something like the following
code:

  int dirfd = open("/path/to/dir", O_DIRECTORY);
  int fd = openat(dirfd, "file", O_PATH | O_NOFOLLOW);
  char *context;
  fgetfilecon(fd, &context);

This change was motivated by a comment in
https://android-review.googlesource.com/#/c/152680/1/toys/posix/ls.c

Change-Id: Ic0cdf9f9dd0e35a63b44a4c4a08400020041eddf
diff --git a/tests/Android.mk b/tests/Android.mk
index 0f3454c..37f7b4f 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -107,6 +107,7 @@
     sys_time_test.cpp \
     sys_types_test.cpp \
     sys_vfs_test.cpp \
+    sys_xattr_test.cpp \
     system_properties_test.cpp \
     time_test.cpp \
     uchar_test.cpp \