[NVPTX] error: no libcall available for fexp

Reproducer: https://godbolt.org/z/raG6exoof
Testcase:
```llvm
target datalayout = "e-p6:32:32-i64:64-i128:128-i256:256-v16:16-v32:32-n16:32:64"
target triple = "nvptx64-unknown-nvidiacl"

define float @_Z10native_expf() {
%1 = tail call float @llvm.exp.f32(float 0x7FF8000000000000)
ret float %1
}

; Function Attrs: nocallback nocreateundeforpoison nofree nosync nounwind speculatable willreturn memory(none)
declare float @llvm.exp.f32(float) #0

attributes #0 = { nocallback nocreateundeforpoison nofree nosync nounwind speculatable willreturn memory(none) }
```

I met this crash when I compiled an OpenCL program and linked it with libclc via LLVM Bitcode.