s390: Add __attribute_const__ to ffs()-family implementations

While tracking down a problem where constant expressions used by
BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
initializer was convincing the compiler that it couldn't track the state
of the prior statically initialized value. Tracing this down found that
ffs() was used in the initializer macro, but since it wasn't marked with
__attribute__const__, the compiler had to assume the function might
change variable states as a side-effect (which is not true for ffs(),
which provides deterministic math results).

Add missing __attribute_const__ annotations to S390's implementations of
ffs(), __ffs(), fls(), and __fls() functions. These are pure mathematical
functions that always return the same result for the same input with no
side effects, making them eligible for compiler optimization.

Build tested ARCH=s390 defconfig with GCC s390x-linux-gnu 14.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Acked-by: Heiko Carstens <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

arch/s390/include/asm/bitops.h

···
  *
  * Undefined if no bit exists, so code should check against 0 first.
  */
-static inline unsigned long __ffs(unsigned long word)
+static inline __attribute_const__ unsigned long __ffs(unsigned long word)
 {
 	return __flogr(-word & word) ^ (BITS_PER_LONG - 1);
 }
···
  * This is defined the same way as the libc and
  * compiler builtin ffs routines (man ffs).
  */
-static inline int ffs(int word)
+static inline __attribute_const__ int ffs(int word)
 {
 	unsigned long mask = 2 * BITS_PER_LONG - 1;
 	unsigned int val = (unsigned int)word;
···
  *
  * Undefined if no set bit exists, so code should check against 0 first.
  */
-static inline unsigned long __fls(unsigned long word)
+static inline __attribute_const__ unsigned long __fls(unsigned long word)
 {
 	return __flogr(word) ^ (BITS_PER_LONG - 1);
 }
···
  * set bit if value is nonzero. The last (most significant) bit is
  * at position 64.
  */
-static inline int fls64(unsigned long word)
+static inline __attribute_const__ int fls64(unsigned long word)
 {
 	unsigned long mask = 2 * BITS_PER_LONG - 1;
 
···
  * This is defined the same way as ffs.
  * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
  */
-static inline int fls(unsigned int word)
+static inline __attribute_const__ int fls(unsigned int word)
 {
 	return fls64(word);
 }