use atomics for loading ICD loader handle vs. C++ static initialization

[bashbaug]

Possible fix for #17

Description of Changes

Uses std::atomics when opening a handle to the ICD loader vs. C++ static initialization.

Testing Done

Stepped through a sample that queried platforms and devices and verified that all API calls succeeded and the ICD loader handle was only opened once.

[bashbaug]

@rcombs would you mind taking a look at this to see if it fixes #17 for you? Thanks!

[rcombs]

Apologies for the long delay in getting this tested; I've been having some logistical issues on my end. This appears to be working fine here and the code looks good!