x86: 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 x86's implementations of
variable__ffs(), variable_ffz(), __fls(), variable_ffs(), 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=x86_64 defconfig with GCC gcc 14.2.0.

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

arch/x86/include/asm/bitops.h

···
 					  variable_test_bit(nr, addr);
 }
 
-static __always_inline unsigned long variable__ffs(unsigned long word)
+static __always_inline __attribute_const__ unsigned long variable__ffs(unsigned long word)
 {
 	asm("tzcnt %1,%0"
 		: "=r" (word)
···
 	 (unsigned long)__builtin_ctzl(word) :	\
 	 variable__ffs(word))
 
-static __always_inline unsigned long variable_ffz(unsigned long word)
+static __always_inline __attribute_const__ unsigned long variable_ffz(unsigned long word)
 {
 	return variable__ffs(~word);
 }
···
  *
  * Undefined if no set bit exists, so code should check against 0 first.
  */
-static __always_inline unsigned long __fls(unsigned long word)
+static __always_inline __attribute_const__ unsigned long __fls(unsigned long word)
 {
 	if (__builtin_constant_p(word))
 		return BITS_PER_LONG - 1 - __builtin_clzl(word);
···
 #undef ADDR
 
 #ifdef __KERNEL__
-static __always_inline int variable_ffs(int x)
+static __always_inline __attribute_const__ int variable_ffs(int x)
 {
 	int r;
 
···
  * set bit if value is nonzero. The last (most significant) bit is
  * at position 32.
  */
-static __always_inline int fls(unsigned int x)
+static __always_inline __attribute_const__ int fls(unsigned int x)
 {
 	int r;
 
···
  * at position 64.
  */
 #ifdef CONFIG_X86_64
-static __always_inline int fls64(__u64 x)
+static __always_inline __attribute_const__ int fls64(__u64 x)
 {
 	int bitpos = -1;