riscv: 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 RISC-V's implementations of
variable__ffs(), variable__fls(), and variable_ffs() 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=riscv defconfig with GCC riscv64-linux-gnu 14.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250804164417.1612371-9-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

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