Stop depending on Integer#size to return `sizeof(long)`

There is no guarantee that Integer#size will continue to return
`sizeof(long)` for small integers.

Use the `l!` specifier for Array#pack instead. It is a public
interface that has a direct relationship with the `long` type.

spec/mspec/lib/mspec/helpers/numeric.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'mspec/guards/platform'
 
 def nan_value
@@ -15,11 +16,13 @@ def bignum_value(plus = 0)
 end
 
 def max_long
-  2**(0.size * 8 - 1) - 1
+  long_byte_size = [0].pack('l!').size
+  2**(long_byte_size * 8 - 1) - 1
 end
 
 def min_long
-  -(2**(0.size * 8 - 1))
+  long_byte_size = [0].pack('l!').size
+  -(2**(long_byte_size * 8 - 1))
 end
 
 # This is a bit hairy, but we need to be able to write specs that cover the

spec/ruby/optional/capi/bignum_spec.rb

@@ -7,21 +7,23 @@ def ensure_bignum(n)
   n
 end
 
-full_range_longs = (fixnum_max == 2**(0.size * 8 - 1) - 1)
+full_range_longs = (fixnum_max == max_long)
+max_ulong = begin
+  require 'rbconfig/sizeof'
+  RbConfig::LIMITS['ULONG_MAX']
+rescue LoadError
+  nil
+end
+# If the system doesn't offer ULONG_MAX, assume 2's complement and derive it
+# from LONG_MAX.
+max_ulong ||= 2 * (max_long + 1) - 1
 
 describe "CApiBignumSpecs" do
   before :each do
     @s = CApiBignumSpecs.new
-
-    if full_range_longs
-      @max_long = 2**(0.size * 8 - 1) - 1
-      @min_long = -@max_long - 1
-      @max_ulong = ensure_bignum(2**(0.size * 8) - 1)
-    else
-      @max_long = ensure_bignum(2**(0.size * 8 - 1) - 1)
-      @min_long = ensure_bignum(-@max_long - 1)
-      @max_ulong = ensure_bignum(2**(0.size * 8) - 1)
-    end
+    @max_long = max_long
+    @min_long = min_long
+    @max_ulong = ensure_bignum(max_ulong)
   end
 
   describe "rb_big2long" do