Consistently provide ffs/fls using builtins

Use of compiler builtin ffs/ctz functions will result in optimized
instruction sequences when possible, and fall back to calling a function
provided by the compiler run-time library. We have slowly shifted our
platforms to take advantage of these builtins in 60645781d6 (arm64),
1c76d3a9fb (arm), 9e319462a0 (powerpc, partial).

Some platforms still rely on the libkern implementations of these
functions provided by libkern, namely riscv, powerpc (ffs*, flsll), and
i386 (ffsll and flsll). These routines are slow, as they perform a
linear search for the bit in question. Even on platforms lacking
dedicated bit-search instructions, such as riscv, the compiler library
will provide better-optimized routines, e.g. by using binary search.

Consolidate all definitions of these functions (whether currently using
builtins or not) to libkern.h. This should result in equivalent or
better performing routines in all cases.

One wart in all of this is the existing HAVE_INLINE_F*** macros, which
we use in a few places to conditionally avoid the slow libkern routines.
These aren't easily removed in one commit. For now, provide these
defines unconditionally, but marked for removal after subsequent
cleanup.

Removal of the now unused libkern routines will follow in the next
commit.

Reviewed by:	dougm, imp (previous version)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D40698

sys/arm/include/cpufunc.h

@@ -183,63 +183,6 @@ void cpu_reset		(void) __attribute__((__noreturn__));
 extern int	arm_dcache_align;
 extern int	arm_dcache_align_mask;
 
-
-#define	HAVE_INLINE_FFS
-
-static __inline __pure2 int
-ffs(int mask)
-{
-
-	return (__builtin_ffs(mask));
-}
-
-#define	HAVE_INLINE_FFSL
-
-static __inline __pure2 int
-ffsl(long mask)
-{
-
-	return (__builtin_ffsl(mask));
-}
-
-#define	HAVE_INLINE_FFSLL
-
-static __inline __pure2 int
-ffsll(long long mask)
-{
-
-	return (__builtin_ffsll(mask));
-}
-
-#define	HAVE_INLINE_FLS
-
-static __inline __pure2 int
-fls(int mask)
-{
-
-	return (mask == 0 ? 0 :
-	    8 * sizeof(mask) - __builtin_clz((u_int)mask));
-}
-
-#define	HAVE_INLINE_FLSL
-
-static __inline __pure2 int
-flsl(long mask)
-{
-
-	return (mask == 0 ? 0 :
-	    8 * sizeof(mask) - __builtin_clzl((u_long)mask));
-}
-
-#define	HAVE_INLINE_FLSLL
-
-static __inline __pure2 int
-flsll(long long mask)
-{
-
-	return (mask == 0 ? 0 :
-	    8 * sizeof(mask) - __builtin_clzll((unsigned long long)mask));
-}
 #else	/* !_KERNEL */
 
 static __inline void