[xray] Disable alignas() for thread_local objects on NetBSD

Disable enforcing alignas() for structs that are used as thread_local
data on NetBSD.  The NetBSD ld.so implementation is buggy and does
not enforce correct alignment; however, clang seems to take it for
granted and generates instructions that segv on wrongly aligned objects.
Therefore, disable those alignas() statements on NetBSD until we can
establish a better fix.

Apparently, std::aligned_storage<> does not have any real effect
at the moment, so we can leave it as-is.

Differential Revision: https://reviews.llvm.org/D56000

llvm-svn: 350029