Add suport for RISCV64

[lazyprogrammerio]

Fixes: #93

[vweevers]

I prefer for this to be fixed upstream, so that we don't have to maintain patches here. If it's already fixed upstream (not unlikely, because we use an old version of LevelDB) then we should consider upgrading.

[lazyprogrammerio]

@vweevers thanks for the quick answer. Seems that upstream is not so well maintained, that's why I am trying here first, last commit was 10 years ago on that file: https://github.com/greenaddress/leveldb/commits/master/port/atomic_pointer.h.

[lazyprogrammerio]

@vweevers thanks for the quick answer. Seems that upstream is not so well maintained, that's why I am trying here first, last commit was 10 years ago on that file: https://github.com/greenaddress/leveldb/commits/master/port/atomic_pointer.h.

In upstream, it is possible to have had the issue fixed by this commit, but I have not tried yet. google/leveldb@7d8e41e#diff-c40252d7003a197df627e35eef63549c496b1d55209e09b68789270e41238025