Inability to use a gamma cdf

In version 1.2.5 I was able to have the entire project compile properly

In version 2.0.5, it managed to run through the precompiliaion workflow, but when building the incremental system image, there was an issue:

```
could not load symbol "pgamma":
/usr/local/julia/bin/julia: undefined symbol: pgamma
Stacktrace:
[1] gammacdf
@ ~/.julia/packages/StatsFuns/mQJB7/src/rmath.jl:83 [inlined]
[2] cdf
@ ~/.julia/packages/Distributions/O4ZJg/src/univariates.jl:622 [inlined]
```
This looks like a problem generated by the macro in this file, line 83 where the actual symbol couldn't be loaded
https://github.com/JuliaStats/StatsFuns.jl/blob/master/src/rmath.jl

I would almost say it was a problem with the StatsFuns.jl package, except that PackageCompiler.jl v1.2.5 was able to handle it.

Start by comparing PackageCompiler.jl 1.2.5 and 2.0.5 while building an incremental system image that uses a gamma CDF. Inspect StatsFuns.jl's src/rmath.jl at line 83 and the reported gammacdf/cdf stack trace. Done means the image builds and loads the pgamma symbol without the undefined-symbol error.

Written by the indexing model from the issue text.