xtensa: 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 Xtensa's implementations
of ffs(), __ffs(), fls(), __fls(), 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=xtensa defconfig with GCC xtensa-linux 15.1.0.

Link: KSPP#364 [1]
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250804164417.1612371-15-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: kees

arch/xtensa/include/asm/bitops.h

@@ -37,7 +37,7 @@ static inline unsigned long __cntlz (unsigned long x)
  * bit 0 is the LSB of addr; bit 32 is the LSB of (addr+1).
  */
 
-static inline int ffz(unsigned long x)
+static inline int __attribute_const__ ffz(unsigned long x)
 {
 	return 31 - __cntlz(~x & -~x);
 }
@@ -46,7 +46,7 @@ static inline int ffz(unsigned long x)
  * __ffs: Find first bit set in word. Return 0 for bit 0
  */
 
-static inline unsigned long __ffs(unsigned long x)
+static inline __attribute_const__ unsigned long __ffs(unsigned long x)
 {
 	return 31 - __cntlz(x & -x);
 }
@@ -57,7 +57,7 @@ static inline unsigned long __ffs(unsigned long x)
  * differs in spirit from the above ffz (man ffs).
  */
 
-static inline int ffs(unsigned long x)
+static inline __attribute_const__ int ffs(unsigned long x)
 {
 	return 32 - __cntlz(x & -x);
 }
@@ -67,7 +67,7 @@ static inline int ffs(unsigned long x)
  * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
  */
 
-static inline int fls (unsigned int x)
+static inline __attribute_const__ int fls (unsigned int x)
 {
 	return 32 - __cntlz(x);
 }
@@ -78,7 +78,7 @@ static inline int fls (unsigned int x)
  *
  * 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 31 - __cntlz(word);
 }