PR20721: Don't let UBSan print inaccessible memory

Summary:
UBSan needs to check if memory snippet it's going to print resides
in addressable memory. Similar check might be helpful in ASan with
dump_instruction_bytes option (see http://reviews.llvm.org/D5167).

Instead of scanning /proc/self/maps manually, delegate this check to
the OS kernel: try to write this memory in a syscall and assume that
memory is inaccessible if the syscall failed (e.g. with EFAULT).

Fixes PR20721.

Test Plan: compiler-rt test suite

Reviewers: eugenis, glider

Reviewed By: glider

Subscribers: emaste, ygribov, llvm-commits, glider, rsmith

Differential Revision: http://reviews.llvm.org/D5253