It looks like the c stdlib interfaces for all of the math functions are now being exported as well as the Swift overlay names, e.g. for sin( ) we have all of the following:

sin(Double) -> Double
sin(x: Double) -> Double
sin(x: Float) -> Float
sin(x: Float80) -> Float80
sinf(Float) -> Float
sinl(Float80) -> Float80

In the case of lgamma, the return type of the C libm functions is actually different:

lgamma(Double) -> Double
lgamma(x: Double) -> (Double, Int)
lgamma(x: Float) -> (Float, Int)
lgamma(x: Float80) -> (Float80, Int)
lgammaf(Float) -> Float
lgammal(Float80) -> Float80

which I think causes the bug we're seeing here.