swi-prolog-lite: Avoid to hardcode 64bit platforms

Instead of hardcoding the check of 64bit platforms just reuse LP64PLATFORMS and
ABI (shamelessy stolen from lang/python27/Makefile).

This fixes the build of swi-prolog-lite on aarch64 and probably
other 64bit platforms too.

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/lang/swi-prolog-lite/Makefile,v
retrieving revision 1.26
diff -u -p -r1.26 Makefile
--- Makefile	17 Apr 2018 20:57:19 -0000	1.26
+++ Makefile	6 Sep 2018 18:46:26 -0000
@@ -17,8 +17,8 @@ USE_TOOLS+=		gmake bash:run
 
 .include "../../mk/bsd.prefs.mk"
 
-.if ${MACHINE_ARCH} == alpha || ${MACHINE_ARCH} == sparc64 || \
-	${MACHINE_ARCH} == x86_64
+.if !empty(LP64PLATFORMS:@.PLAT.@${MACHINE_PLATFORM:M${.PLAT.}}@) || \
+    (defined(ABI) && ${ABI} == "64")
 PLIST_SUBST+=		BITS=64
 .else
 PLIST_SUBST+=		BITS=32