glibc 2.15 treats errno as signed in strerror(3).

And the only reason I hadn't done that in bionic is because I wanted to behave
the same as glibc.

Change-Id: I2cf1bf0aac82a748cd6305a2cabbac0790058570

tests/string_test.cpp

@@ -39,7 +39,7 @@ TEST(string, strerror) {
   ASSERT_STREQ("Operation not permitted", strerror(1));
 
   // Invalid.
-  ASSERT_STREQ("Unknown error 4294967295", strerror(-1));
+  ASSERT_STREQ("Unknown error -1", strerror(-1));
   ASSERT_STREQ("Unknown error 1234", strerror(1234));
 }