BUG Fixes cython code for ppc arch (#17201)

* BUG Fixes cython in splitting

* REV Revert

* Revert "REV Revert"

This reverts commit d1cf3b1.

* WIP Uses signed char

* WIP Maybe use signed chars

* WIP const signed chars

sklearn/ensemble/_hist_gradient_boosting/splitting.pyx

@@ -134,7 +134,7 @@ cdef class Splitter:
         const unsigned int [::1] n_bins_non_missing
         unsigned char missing_values_bin_idx
         const unsigned char [::1] has_missing_values
-        const char [::1] monotonic_cst
+        const signed char [::1] monotonic_cst
         unsigned char hessians_are_constant
         Y_DTYPE_C l2_regularization
         Y_DTYPE_C min_hessian_to_split
@@ -150,7 +150,7 @@ cdef class Splitter:
                  const unsigned int [::1] n_bins_non_missing,
                  const unsigned char missing_values_bin_idx,
                  const unsigned char [::1] has_missing_values,
-                 const char [::1] monotonic_cst,
+                 const signed char [::1] monotonic_cst,
                  Y_DTYPE_C l2_regularization,
                  Y_DTYPE_C min_hessian_to_split=1e-3,
                  unsigned int min_samples_leaf=20,
@@ -410,7 +410,7 @@ cdef class Splitter:
             split_info_struct split_info
             split_info_struct * split_infos
             const unsigned char [::1] has_missing_values = self.has_missing_values
-            const char [::1] monotonic_cst = self.monotonic_cst
+            const signed char [::1] monotonic_cst = self.monotonic_cst
 
         with nogil:
 
@@ -496,7 +496,7 @@ cdef class Splitter:
             Y_DTYPE_C sum_gradients,
             Y_DTYPE_C sum_hessians,
             Y_DTYPE_C value,
-            char monotonic_cst,
+            signed char monotonic_cst,
             Y_DTYPE_C lower_bound,
             Y_DTYPE_C upper_bound,
             split_info_struct * split_info) nogil:  # OUT
@@ -610,7 +610,7 @@ cdef class Splitter:
             Y_DTYPE_C sum_gradients,
             Y_DTYPE_C sum_hessians,
             Y_DTYPE_C value,
-            char monotonic_cst,
+            signed char monotonic_cst,
             Y_DTYPE_C lower_bound,
             Y_DTYPE_C upper_bound,
             split_info_struct * split_info) nogil:  # OUT
@@ -723,7 +723,7 @@ cdef inline Y_DTYPE_C _split_gain(
         Y_DTYPE_C sum_gradient_right,
         Y_DTYPE_C sum_hessian_right,
         Y_DTYPE_C loss_current_node,
-        char monotonic_cst,
+        signed char monotonic_cst,
         Y_DTYPE_C lower_bound,
         Y_DTYPE_C upper_bound,
         Y_DTYPE_C l2_regularization) nogil:
@@ -820,4 +820,4 @@ cpdef inline Y_DTYPE_C compute_node_value(
     elif value > upper_bound:
         value = upper_bound
 
-    return value
+    return value