Add gmp-arch and udiv_qrnnd

Add gmp-arch and udiv_qrnnd

To enable “longlong.h” removal, the udiv_qrnnd is moved to a gmp-arch.h
file.  It allows each architecture to implement its own arch-specific
optimizations. The generic implementation now uses a static inline,
which provides better type checking than the GNU extension to cast the
asm constraint (and it works better with clang).

Most of the architecture uses the generic implementation, which is
expanded from a macro, except for alpha, x86, m68k, sh, and sparc.
I kept that alpha, which uses out-of-the-line implementations and x86,
where there is no easy way to use the div{q} instruction from C code.
For the rest, the compiler generates good enough code.

The hppa also provides arch-specific implementations, but they are not
routed in “longlong.h” and thus never used.