mpi/longlong.h: x86-64: use tzcnt instruction for trailing zeros

* mpi/longlong.h [__x86_64__] (count_trailing_zeros): Add 'rep' prefix
for 'bsfq'.

"rep;bsf" aka "tzcnt" is new instruction with well defined operation
on zero input and as result is faster on new CPUs. On old CPUs, "tzcnt"
functions as old "bsf" with undefined behaviour on zero input.

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