Use the form with underscores to define the '__deprecated' macro to avoid
collisions with potentially defined macros, as suggested in gcc docs
(https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax).

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>

Introduce a macro to mark functions as deprecated.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>

- 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: I3e3d8d992052264d2a02489034ae4c03bf0f5512