mpi/longlong: prevent optimization of carry instructions to branches

* mpi/longlong.h: Include "const-time.h"
(add_ssaaaa, sub_ddmmss): Prevent optimization of carry handling to
conditional branches in generic variant of double width addition and
subtraction as was seen with GCC on riscv64.
(umul_ppmm): Avoid conditional branch in generic 16x16=>32bit
multiplication version of umul_ppmm.
* src/const-time.h (CT_DEOPTIMIZE_VAR): New.

RISC-V has "sltu" instruction for generating carry value and
generic version of add_ssaaaa and sub_ddmmss typically used this
instruction. However, sometimes compiler gets too clever and
instead generates code with conditional branch, which is not good
for constant time code. Commit changes add_ssaaaaa and sub_ddmmss
to clobber high word of calculation in a way that prevents such
optimizations.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>