patch 8.2.0271: the "num64" feature is available everywhere

Problem:    The "num64" feature is available everywhere and building without
            it causes problems.
Solution:   Graduage the "num64" feature. (James McCoy, closes #5650)
diff --git a/src/testdir/test_vimscript.vim b/src/testdir/test_vimscript.vim
index a75e5f6..b16b05b 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -1416,10 +1416,6 @@
 "-------------------------------------------------------------------------------
 
 func Test_num64()
-    if !has('num64')
-	return
-    endif
-
     call assert_notequal( 4294967296, 0)
     call assert_notequal(-4294967296, 0)
     call assert_equal( 4294967296,  0xFFFFffff + 1)
@@ -1692,27 +1688,15 @@
     " Test special cases: division or modulus with 0.
     let x = 1
     let x /= 0
-    if has('num64')
-        call assert_equal(0x7FFFFFFFFFFFFFFF, x)
-    else
-        call assert_equal(0x7fffffff, x)
-    endif
+    call assert_equal(0x7FFFFFFFFFFFFFFF, x)
 
     let x = -1
     let x /= 0
-    if has('num64')
-        call assert_equal(-0x7FFFFFFFFFFFFFFF, x)
-    else
-        call assert_equal(-0x7fffffff, x)
-    endif
+    call assert_equal(-0x7FFFFFFFFFFFFFFF, x)
 
     let x = 0
     let x /= 0
-    if has('num64')
-        call assert_equal(-0x7FFFFFFFFFFFFFFF - 1, x)
-    else
-        call assert_equal(-0x7FFFFFFF - 1, x)
-    endif
+    call assert_equal(-0x7FFFFFFFFFFFFFFF - 1, x)
 
     let x = 1
     let x %= 0