[ASan] Fix asan_preload_test-2 on PowerPC64 Linux

[foad]

Summary

This test failed because clang compiled the call to memset() into a
single sth instruction, instead of a call. Fix it by using write() instead
of memset().

[eugenis]

Would it work if you change memset to something that is unlikely to be optimized, like write(1, p, 2) ?

[foad]

In D6776#104155, @eugenis wrote:

Would it work if you change memset to something that is unlikely to be optimized, like write(1, p, 2) ?

Yes, that works for me. But it'll write some nasty characters to stdout if the test fails.

[foad]

Use write instead of memset.

[eugenis]

Right. Either way is fine with me.

test/asan/TestCases/Linux/asan_preload_test-2.cc

 // Test that preloaded runtime works with unsanitized executables.
 //
 // RUN: %clangxx %s -o %t
 // RUN: LD_PRELOAD=%shared_libasan not %run %t 2>&1 | FileCheck %s
 
 // REQUIRES: asan-dynamic-runtime
 
 // This way of setting LD_PRELOAD does not work with Android test runner.
 // REQUIRES: not-android
 
 #include <stdlib.h>
 
-extern "C" void *memset(void *p, int val, size_t n);
+extern "C" ssize_t write(int fd, const void *buf, size_t count);
 
 void do_access(void *p) {
   // CHECK: AddressSanitizer: heap-buffer-overflow
-  memset(p, 0, 2);
+  write(1, p, 2);
 }
 
 int main(int argc, char **argv) {
   void *p = malloc(1);
   do_access(p);
   return 0;
 }