Allow ThreadLauncher::LaunchThread() to specify a minimum stack byte size when launching threads.

This defaults to zero, which means to use the system default.

NOTE: Windows will need to implement this.

<rdar://problem/18644448>



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@219821 91177308-0d34-0410-b5e6-96231b3b80d8