tsan: re-enable mmap_stress.cpp test

The comment says it was flaky in 2016,
but it wasn't possible to debug it back then.
Re-enable the test at least on linux/x86_64.
It will either work, or at least we should
see failure output from lit today.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D104592