[ASan] Allow to atomically modify malloc_context_size at runtime.

Summary:
Introduce __asan::malloc_context_size atomic that is used to determine
required malloc/free stack trace size. It is initialized with
common_flags()->malloc_context_size flag, but can later be overwritten
at runtime (e.g. when ASan is activated / deactivated).

Test Plan: regression test suite

Reviewers: kcc, eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6645