[libc] Update errno and strto* files for compatibility

Most of the strto* functions are only stubbed out, but they're there for
things that link to them but don't call them.

src/libraries/libc/arch/amd64/errno.cpp

@@ -0,0 +1,10 @@
+#include <arch/amd64/errno.h>
+
+static int errno_value = 0;
+
+int *
+__errno_location()
+{
+    // TODO: thread-local errno
+    return &errno_value;
+}