crypto: serpent - improve __serpent_setkey with UBSAN

When UBSAN is enabled, we get a very large stack frame for
__serpent_setkey, when the register allocator ends up using more registers
than it has, and has to spill temporary values to the stack. The code
was originally optimized for in-order x86-32 CPU implementations using
older compilers, but it now runs into a highly suboptimal case on all
CPU architectures, as seen by this warning:

crypto/serpent_generic.c: In function '__serpent_setkey':
crypto/serpent_generic.c:436:1: error: the frame size of 2720 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]

Disabling -fsanitize=alignment would avoid that warning, presumably the
option turns off a optimization step that is required for getting the
register allocation right, but there is no easy way to do that on gcc-7
(gcc-8 introduces a function attribute for this).

I tried to figure out a way to modify the source code instead, and noticed
that the two stages of the setkey() function (keyiter ...