BUILD: remove obsolete support for -mregparm / USE_REGPARM

This used to be a minor optimization on ix86 where registers are scarce
and the calling convention not very efficient, but this platform is not
relevant enough anymore to warrant all this dirt in the code for the sake
of saving 1 or 2% of performance. Modern platforms don't use this at all
since their calling convention already defaults to using several registers
so better get rid of this once for all.
diff --git a/ebtree/eb64tree.h b/ebtree/eb64tree.h
index 6d0d039..9c8feeb 100644
--- a/ebtree/eb64tree.h
+++ b/ebtree/eb64tree.h
@@ -110,12 +110,12 @@
  * The following functions are not inlined by default. They are declared
  * in eb64tree.c, which simply relies on their inline version.
  */
-REGPRM2 struct eb64_node *eb64_lookup(struct eb_root *root, u64 x);
-REGPRM2 struct eb64_node *eb64i_lookup(struct eb_root *root, s64 x);
-REGPRM2 struct eb64_node *eb64_lookup_le(struct eb_root *root, u64 x);
-REGPRM2 struct eb64_node *eb64_lookup_ge(struct eb_root *root, u64 x);
-REGPRM2 struct eb64_node *eb64_insert(struct eb_root *root, struct eb64_node *new);
-REGPRM2 struct eb64_node *eb64i_insert(struct eb_root *root, struct eb64_node *new);
+struct eb64_node *eb64_lookup(struct eb_root *root, u64 x);
+struct eb64_node *eb64i_lookup(struct eb_root *root, s64 x);
+struct eb64_node *eb64_lookup_le(struct eb_root *root, u64 x);
+struct eb64_node *eb64_lookup_ge(struct eb_root *root, u64 x);
+struct eb64_node *eb64_insert(struct eb_root *root, struct eb64_node *new);
+struct eb64_node *eb64i_insert(struct eb_root *root, struct eb64_node *new);
 
 /*
  * The following functions are less likely to be used directly, because their