Create local C library implementation (1/2)

- This change is split into two separate patches in order to
  simplify the history as interpreted by 'git'. The split is
  between the move/rename and addition of new files.
- Remove dependency on toolchain C library headers and functions in
  order to ensure behavioural compatibility between toolchains.
- Use FreeBSD as reference for C library implementation.
- Do not let GCC use default library include paths.
- Remove unused definitions in modified headers and implementations.
- Move C library files to 'lib/stdlib' and 'include/stdlib'.
- Break std.c functions out into separate files.

Change-Id: I91cddfb3229775f770ad781589670c57d347a154

lib/semihosting/semihosting.c

@@ -29,7 +29,6 @@
  */
 
 #include <assert.h>
-#include <stdlib.h>
 #include <string.h>
 #include <errno.h>
 #include <semihosting.h>