Transform ProcessLauncherLinux to ProcessLauncherPosixFork

Summary:
Use ProcessLauncherPosixFork in Linux and NetBSD.

Changes to ProcessLauncherLinux:
 - Limit personality.h and ASLR code to Linux.
 - Reuse portable ptrace(2) PT_TRACE_ME operation available on Linux and BSDs.
 - Limit ETXTBSY error path from execve(2) to Linux.
 - In LaunchProcess declaration change virtual to override.

This code should be readily available for FreeBSD.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, clayborg, labath, emaste

Reviewed By: labath

Subscribers: danalbert, srhines, mgorny, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D29347

llvm-svn: 293768