bitmap: don't assume compiler evaluates small mem*() builtins calls

Intel kernel bot triggered the build bug on ARC architecture that
in fact is as follows:

	DECLARE_BITMAP(bitmap, BITS_PER_LONG);

	bitmap_clear(bitmap, 0, BITS_PER_LONG);
	BUILD_BUG_ON(!__builtin_constant_p(*bitmap));

which can be expanded to:

	unsigned long bitmap[1];

	memset(bitmap, 0, sizeof(*bitmap));
	BUILD_BUG_ON(!__builtin_constant_p(*bitmap));

In most cases, a compiler is able to expand small/simple mem*()
calls to simple assignments or bitops, in this case that would mean:

	unsigned long bitmap[1] = { 0 };

	BUILD_BUG_ON(!__builtin_constant_p(*bitmap));

and on most architectures this works, but not on ARC, despite having
-O3 for every build.
So, to make this work, in case when the last bit to modify is still
within the first long (small_const_nbits()), just use plain
assignments for the rest of bitmap_*() functions which still use
mem*(), but didn't receive such compile-time optimizations yet.
This doesn't have the same coverage as compilers provide, but at
least something to start:

text: add/remove: 3/7 grow/shrink: 43/78 up/down: 1848/-3370 (-1546)
data: add/remove: 1/11 grow/shrink: 0/8 up/down: 4/-356 (-352)

notably cpumask_*() family when NR_CPUS <= BITS_PER_LONG:

netif_get_num_default_rss_queues              38       4     -34
cpumask_copy                                  90       -     -90
cpumask_clear                                146       -    -146

and the abovementioned assertion started passing.

Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>