Fix x86 system calls made from ELF preinit.

Used by CFI, so broke cfi_test#early_init@x86, but I've added a specific
test for this (and a similar test for getauxval from preinit, which this
patch does not fix).

Bug: http://b/35885875
Test: ran tests
Change-Id: I43885bedfb88c0a26b4474bd3c27a87dec7bbc97
diff --git a/tests/libs/libs_utils.h b/tests/libs/libs_utils.h
index f11cbe7..7dae241 100644
--- a/tests/libs/libs_utils.h
+++ b/tests/libs/libs_utils.h
@@ -17,13 +17,11 @@
 #ifndef LIBS_UTILS_H
 #define LIBS_UTILS_H
 
+#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 
-#define CHECK(x)                                                            \
-  do {                                                                      \
-    fprintf(stderr, "CHECK(" #x ") failed at %s:%d\n", __FILE__, __LINE__); \
-    if (!(x)) abort();                                                      \
-  } while (0)
+#define CHECK(e) \
+    ((e) ? static_cast<void>(0) : __assert2(__FILE__, __LINE__, __PRETTY_FUNCTION__, #e))
 
 #endif  // LIBS_UTILS_H