LibC: Add ELAST errno macro

The ELAST macro is used on many systems to refer to the largest possible
valid errno value. LLVM's libc++ uses errno values of ELAST+1 and
ELAST+2 internally, and defines an arbitrary fallback value for
platforms which don't have the macro. This means that it's possible for
their internal errno numbers could coincide with values we actually use,
which would be a very bad thing.

Userland/Libraries/LibC/errno_numbers.h

@@ -160,3 +160,4 @@ enum ErrnoCode {
 };
 
 #define EWOULDBLOCK EAGAIN
+#define ELAST EMAXERRNO