ARM: 8785/1: use compiler built-ins for ffs and fls

On ARMv5 and above, it is beneficial to use compiler built-ins such as
__builtin_ffs() and __builtin_ctzl() to implement ffs(), __ffs(), fls()
and __fls(). The compiler does inline the clz instruction and even the
rbit instruction when available, or provide a constant value when
possible. On ARMv4 the compiler calls out to helper functions for those
built-ins so it is best to keep the open coded versions in that case.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>