[libc] Use __builtin_ctzll instead of __builtin_ctzl in elements_x86.h.

__builtin_ctzl takes an unsigned long argument which need not be 64-bit
long on all platforms. Using __builtin_ctzll, which takes an unsigned
long long argument, ensures that 64-bit values will be handled on a
wider range of platforms.

Without this change, the test corresponding to M512 fails in Windows.

Reviewed By: gchatelet

Differential Revision: https://reviews.llvm.org/D104897