m68k: 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 M68K's implementations
of ffs(), __ffs(), fls(), __fls(), and ffz() 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=m68k defconfig with GCC m68k-linux-gnu 14.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Acked-by: Geert Uytterhoeven <[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/m68k/include/asm/bitops.h

···
  * ffz = Find First Zero in word. Undefined if no zero exists,
  * so code should check against ~0UL first..
  */
-static inline unsigned long ffz(unsigned long word)
+static inline unsigned long __attribute_const__ ffz(unsigned long word)
 {
 	int res;
 
···
  */
 #if (defined(__mcfisaaplus__) || defined(__mcfisac__)) && \
 	!defined(CONFIG_M68000)
-static inline unsigned long __ffs(unsigned long x)
+static inline __attribute_const__ unsigned long __ffs(unsigned long x)
 {
 	__asm__ __volatile__ ("bitrev %0; ff1 %0"
 		: "=d" (x)
···
 	return x;
 }
 
-static inline int ffs(int x)
+static inline __attribute_const__ int ffs(int x)
 {
 	if (!x)
 		return 0;
···
  *	the libc and compiler builtin ffs routines, therefore
  *	differs in spirit from the above ffz (man ffs).
  */
-static inline int ffs(int x)
+static inline __attribute_const__ int ffs(int x)
 {
 	int cnt;
 
···
 	return 32 - cnt;
 }
 
-static inline unsigned long __ffs(unsigned long x)
+static inline __attribute_const__ unsigned long __ffs(unsigned long x)
 {
 	return ffs(x) - 1;
 }
···
 /*
  *	fls: find last bit set.
  */
-static inline int fls(unsigned int x)
+static inline __attribute_const__ int fls(unsigned int x)
 {
 	int cnt;
 
···
 	return 32 - cnt;
 }
 
-static inline unsigned long __fls(unsigned long x)
+static inline __attribute_const__ unsigned long __fls(unsigned long x)
 {
 	return fls(x) - 1;
 }