Streamline libsel4 and remove its libc dependencies.

There are now separate libs for benchmark, assert, printf, putchar
start/stop:

      libs/libsel4benchmark
      libs/libsel4assert
      libs/libsel4printf
      libs/libsel4putchar
      libs/libsel4startstop

The primary changes are introducing sel4/sel4.h and removing std* types
plus porting assert and IO code from the kernel to libsel4assert,
libsel4printf, libsel4putchar.

This means the code within libsel4 and the newlibs do not overload any
typical libc entities. Instead the libraries use types like
seL4_Uint32 ... instead of uint32_t. And printf is now seL4_Printf and
assert is seL4_Assert ....

Finally, the only file modified that effects kernel code is
kernel/tools/bitfield_gen.py. It needed to be modified as it generates
files for both kernel and user space. And for user space the generated code
(types_gen.h) needed to use the new types and asserts. The changes should
not change what is generated for the kernel and I did a comparison of
kernel_final.{c|s} before and after my change and the only differences
were time stamps.

Bug: #15 Streamline kernel/libsel4 and remove its libc dependencies