Fix boost-context compatibility for versions above 1.52

Summary:
For 1.54 code path, we would never initialize mainContext_ (the context pointer would be null),
so jump_fcontext would fail.
We need to store mainContext_ as the context struct directly, and support jumping to/from
a direct struct.

Test Plan: tested open source build with 1.54 and 1.56 boost

Reviewed By: pavlo@fb.com

Subscribers: alikhtarov

FB internal diff: D1639842