Add GNU-compatible strerror_r.

We already had the POSIX strerror_r, but some third-party code defines
_GNU_SOURCE and expects to get the GNU strerror_r instead.

This exposed a bug in the libc internal logging functions where unlike
their standard brethren they wouldn't return the number of bytes they'd
have liked to have written.

Bug: 16243479
Change-Id: I1745752ccbdc569646d34f5071f6df2be066d5f4
diff --git a/tests/libc_logging_test.cpp b/tests/libc_logging_test.cpp
index 950161e..d4ceded 100644
--- a/tests/libc_logging_test.cpp
+++ b/tests/libc_logging_test.cpp
@@ -176,3 +176,15 @@
   GTEST_LOG_(INFO) << "This test does nothing.\n";
 #endif // __BIONIC__
 }
+
+TEST(libc_logging, buffer_overrun) {
+#if defined(__BIONIC__)
+  char buf[BUFSIZ];
+  ASSERT_EQ(11, __libc_format_buffer(buf, sizeof(buf), "hello %s", "world"));
+  EXPECT_STREQ("hello world", buf);
+  ASSERT_EQ(11, __libc_format_buffer(buf, 8, "hello %s", "world"));
+  EXPECT_STREQ("hello w", buf);
+#else // __BIONIC__
+  GTEST_LOG_(INFO) << "This test does nothing.\n";
+#endif // __BIONIC__
+}