[ASan] Fix asan_preload_test-2 on PowerPC64 Linux

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().

Reviewers: kcc, samsonov, garious, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6776