[llvm-exegesis] Switch to using PTRACE_ATTACH instead of PTRACE_SEIZE

This patch switches from using PTRACE_SEIZE within the subprocess
benchmark runner for llvm-exegesis as PTRACE_SEIZE was introduced in
Linux kernel 3.4. Some LLVM users were reporting build failures as they
are using Kernel versions older than 3.4 (such as on CentOS/RHEL 6),
hence the patch.