fix whatever is wrong with normal_distribution

[jaredhoberock]

Seems to be untested and broken.

[jaredhoberock]

The problem seems to be the use of the non-standard math function erfcinv, which makes it non-portable.

An alternative is the "polar method" which generates two points at once, and caches the 2nd result to use on the next invocation. We could do this when the compiler is not nvcc, but it would involve refactoring the code to accomodate the cache.