[ASan] Skip ptrace test on non-x86 targets

Summary:
Address sanitization of ptrace(2) is only implemented for x86, so skip
the test on other targets.

Reviewers: kcc, eugenis, earthdok, samsonov

Reviewed By: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6215

llvm-svn: 221777