boost: module context affected by GCC bug 64735

boost: module context affected by GCC bug 64735

Building boosts module context is affected by GCC bug 64735 [1]:

```
./boost/context/execution_context_v2.hpp:298:31: error: 'exception_ptr' is not a member of 'std'
     auto p = std::make_tuple( std::exception_ptr{}, std::move( data) );
```

This is because `exception_ptr` from libstdc++ is not available for
architectures not supporting always lock-free atomic ints.

Note, that this bug is fixed in GCC 7.

Fixes:
http://autobuild.buildroot.net/results/f4e/f4e8a3727f19f7df4a1c45efe3603422fafd4af5/

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64735

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>