x86_64 has an incorrect ffs() implementation.  The asm uses "g" instead
of "rm" for the bsfl instruction.  (This was spotted by Yuri Per.)

bsfl does not accept constant values but only memory or register ones.
On i386 the correct "rm" is used.

This causes NTFS build to fail as gcc optimizes a variable into a
constant and ffs() then fails to assemble.