Process.warmup: invoke malloc_trim if available

Similar to releasing free GC pages, releasing free malloc pages
reduce the amount of page faults post fork.

configure.ac

@@ -2078,6 +2078,7 @@ AC_CHECK_FUNCS(lstat)
 AC_CHECK_FUNCS(lutimes)
 AC_CHECK_FUNCS(malloc_usable_size)
 AC_CHECK_FUNCS(malloc_size)
+AC_CHECK_FUNCS(malloc_trim)
 AC_CHECK_FUNCS(mblen)
 AC_CHECK_FUNCS(memalign)
 AC_CHECK_FUNCS(memset_s)

gc.c

@@ -9740,6 +9740,10 @@ rb_gc_prepare_heap(void)
     rb_objspace_each_objects(gc_set_candidate_object_i, NULL);
     gc_start_internal(NULL, Qtrue, Qtrue, Qtrue, Qtrue, Qtrue);
     free_empty_pages();
+
+#if defined(HAVE_MALLOC_TRIM) && !defined(RUBY_ALTERNATIVE_MALLOC_HEADER)
+    malloc_trim(0);
+#endif
 }
 
 static int

process.c

@@ -8789,6 +8789,7 @@ static VALUE rb_mProcID_Syscall;
  *  * Precomputes the coderange of all strings.
  *  * Frees all empty heap pages and increments the allocatable pages counter
  *    by the number of pages freed.
+ *  * Invoke +malloc_trim+ if available to free empty malloc pages.
  */
 
 static VALUE