[C++] Arrow version 5 or later fails to compile/link with gcc version 7 (or earlier)

### Describe the bug, including details regarding any error messages, version, and platform.

When linking the shared library with gcc 7 I get several errors like this:

```
_ZN5arrow6ResultISt10unique_ptrINS_7compute11KernelStateESt14default_deleteIS3_EEEC2ERKNS_6StatusE.part.688' referenced in section `.text' of CMakeFiles/arrow_objlib.dir/compute/kernels/scalar_string_ascii.cc.o: defined in discarded section `.text._ZN5arrow6ResultISt10unique_ptrINS_7compute11KernelStateESt14default_deleteIS3_EEEC2ERKNS_6StatusE.part.688[_ZN5arrow6ResultISt10unique_ptrINS_7compute11KernelStateESt14default_deleteIS3_EEEC5ERKNS_6StatusE]' of CMakeFiles/arrow_objlib.dir/compute/kernels/scalar_string_ascii.cc.o`
```

### Component(s)

C++

Start by reproducing the shared-library build and link with GCC 7 or earlier, using the linker error in the issue as the initial failure signal. Investigate the Arrow 5+ C++ build configuration and generated objects involved in the discarded-section error. Done means the shared library compiles and links successfully with the affected GCC versions.

Written by the indexing model from the issue text.