[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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@221777 91177308-0d34-0410-b5e6-96231b3b80d8