tile: use asm-generic/bitops/builtin-*.h

The definisions of __ffs(), __fls(), and ffs() for tile are almost same
as asm-generic/bitops-*.h.  The only difference is that it is defined
as __always_inline or inline.  So this switches to use those headers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com> [moved #includes to end]