alloc tests: hook posix_memalign & reallocf (#782)

[weissi]

Motivation:

Swift started using posix_memalign very recently (Jan 2019) so we need
to hook that too to get correct results.

Modifications:

Hook posix_memalign, reallocf & valloc

Result:

alloc tests should pass again

(cherry picked from commit de8d3e2)

[Lukasa]

We're ignoring the Swift 5 test failure here: @weissi assures me that he expected that failure and that it's not relevant right now.

[weissi]

5.0 failure is expected because the docker file only comes in #952 (which can't go in ahead of this one because this backports the allocation counter fixes for Swift 5 from master)