Add quadword math library

GCC implicitly calls functions like __adddi3 to handle arithmetics on long long.
Newlib depends on these functions in its printf() but expects them to be present.
Since every function is defined in its own compiling unit, only the function that are actually used will wind up in the binary.

There is no need for doxygen comments; you are not supposed to call these functions directly.

Copied from http://ftp3.usa.openbsd.org/pub/OpenBSD/src/lib/libc/quad/ which is curtesy of the OpenBSD project (3 clause BSD license).