Set correct shared library name.

Override the arch setting to remove OpenBSD version from it,
so ports don't have to be bumped when OpenBSD version changes.

Index: configure
--- configure.orig
+++ configure
@@ -29855,7 +29855,7 @@ fi
 	 ;; #(
   openbsd*|mirbsd*) :
 
-	LIBRUBY_SO='lib$(RUBY_SO_NAME).$(SOEXT).$(MAJOR).'`expr ${MINOR} \* 10 + ${TEENY}`
+	LIBRUBY_SO='lib$(RUBY_SO_NAME).so.'${LIBruby30_VERSION}
 	 ;; #(
   solaris*) :
 
@@ -31424,7 +31424,7 @@ _ACEOF
 
 else
 
-    arch="${target_cpu}-${target_os}"
+    arch="${target_cpu}-openbsd"
     cat >>confdefs.h <<_ACEOF
 #define RUBY_PLATFORM "$arch"
 _ACEOF
