tsan: align globals and stack on 8-bytes

[dvyukov]

Tsan shadow granularity is 8-bytes. When several variables are collocated in the same 8-bytes, it can cause false negatives due to shadow slot eviction. In http://reviews.llvm.org/D14434 Kuba Brecka proposed to align globals on 8-bytes to mitigate this. We could also align stack variables.
Aligning stack variables will cause stack usage increase and can lead to crashes if program uses small stacks.
I am not sure whether it is worth it or not. In http://reviews.llvm.org/D14434 tsan still detected the race occasionally. And heap and structs will be left un-aligned.

[morehouse]

@dvyukov: Any updates? Do you think this is worth the effort?

[dvyukov]

I would try this for globals. Smalls globals can cause false sharing on tsan shadow, and tests showed that detection is very flaky when several variables are collocated in the same granule.