bitops: provide an inline implementation of find_first_bit

bitops: provide an inline implementation of find_first_bit

find_first_bit has started to be used heavily in TCG code. The current
implementation based on find_next_bit is not optimal and can't be
optimized be the compiler if the bit array has a fixed size, which is
the case most of the time.

This new implementation does not use find_next_bit and is yet small
enough to be inlined.

Cc: Corentin Chary <corentin.chary@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>