tsan: fix signal_longjmp test in debug mode
In debug mode tsan checks that user accesses
access user memory. NULL is not user memory.
So the test fails. Allocate real inaccessible
memory for the test.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218069 91177308-0d34-0410-b5e6-96231b3b80d8