Call arity testcase for #3924

nice numbers coming from these micro-benchmarks.

testsuite/tests/callarity/perf/Makefile

+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk

testsuite/tests/callarity/perf/T3924.hs

+f2 :: Int -> Int -> Int
+f2 x1 = if x1 == 0 then (\x0 -> x0) else let
+     y = x1 - 1
+     in f3 y y
+f3 :: Int -> Int -> Int -> Int
+f3 x2 = if x2 == 0 then f2 else let
+     y = x2 - 1
+     in f4 y y
+f4 :: Int -> Int -> Int -> Int -> Int
+f4 x3 = if x3 == 0 then f3 else let
+     y = x3 - 1
+     in \x2 x1 x0 -> f4 y x2 x1 (y + x0)
+main = print (f2 100 0)

testsuite/tests/callarity/perf/T3924.stdout

+3921225

testsuite/tests/callarity/perf/all.T

+test('T3924',
+     [stats_num_field('bytes allocated', (51480, 5)),
+          # previously, without call-arity: 22326544
+          # 2014-01-18: 51480  (amd64/Linux)
+      only_ways(['normal'])
+      ],
+     compile_and_run,
+     ['-O'])