Make it possible to build bionic with -I instead of -isystem.

Various things:

* work around -Wnullability-completeness.
* use C++ casts in C++ and C casts in C.
* stop using attributes clang doesn't support (such as `warning`).
* remove duplicate definitions of XATTR_CREATE and XATTR_REPLACE.

Change-Id: I07649e46275b28a23ca477deea119fe843999533
diff --git a/libc/include/stdint.h b/libc/include/stdint.h
index a66e21e..0ad27f1 100644
--- a/libc/include/stdint.h
+++ b/libc/include/stdint.h
@@ -32,7 +32,7 @@
 #include <bits/wchar_limits.h>
 #include <stddef.h>
 
-typedef __signed char __int8_t;
+typedef signed char __int8_t;
 typedef unsigned char __uint8_t;
 typedef short __int16_t;
 typedef unsigned short __uint16_t;