Introduce CheckASLR() in sanitizers

Summary:
At least the ASan, MSan, TSan sanitizers require disabled ASLR on a NetBSD.

Introduce a generic CheckASLR() routine, that implements a check for the
current process. This flag depends on the global or per-process settings.

There is no simple way to disable ASLR in the build process from the
level of a sanitizer or during the runtime execution.

With ASLR enabled sanitizers that operate over the process virtual address
space can misbehave usually breaking with cryptic messages.

This check is dummy for !NetBSD.

Sponsored by <The NetBSD Foundation>

Reviewers: vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: cryptoad, kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D47442

llvm-svn: 333985