Add macros to call the appropriate ffs() function

ffs finds the least significant 1 bit.  POSIX defines it only for
integer arguments, but many implementations have a version for longs as
well.

This commit defines a macro for the appropriate one for this platform,
if any.  Future commits will use this.

perl.h

@@ -3872,12 +3872,18 @@ hint to the compiler that this condition is likely to be false.
                              || __GNUC__ == 3 && __GNUC_MINOR__ >= 4))
 #    define PERL_USE_CLZ(x) __builtin_clz(x)
 #  endif
+#  ifdef HAS_FFS
+#    define PERL_USE_FFS(x)  ffs(x)
+#  endif
 #elif PERL_UINTMAX_SIZE == LONGSIZE
 #  if  __has_builtin(__builtin_clzl)                                        \
    || (defined(__GNUC__) && (   __GNUC__ > 3                                \
                              || __GNUC__ == 3 && __GNUC_MINOR__ >= 4))
 #    define PERL_USE_CLZ(x) __builtin_clzl(x)
 #  endif
+#  ifdef HAS_FFS
+#    define PERL_USE_FFS(x)  ffsl(x)
+#  endif
 #endif
 
 /*