ARROW-8962: [C++] Add explicit implementation for junk values
ARROW-8962: [C++] Add explicit implementation for junk values

When linking the tests with `clang-4.0`, I get

```
Undefined symbols for architecture x86_64:
  "arrow::internal::(anonymous namespace)::StringToFloatConverterImpl::main_junk_value_", referenced from:
      arrow::internal::StringToFloat(char const*, unsigned long, float*) in libarrow.a(value_parsing.cc.o)
      arrow::internal::StringToFloat(char const*, unsigned long, double*) in libarrow.a(value_parsing.cc.o)
  "arrow::internal::(anonymous namespace)::StringToFloatConverterImpl::fallback_junk_value_", referenced from:
      arrow::internal::StringToFloat(char const*, unsigned long, float*) in libarrow.a(value_parsing.cc.o)
      arrow::internal::StringToFloat(char const*, unsigned long, double*) in libarrow.a(value_parsing.cc.o)
ld: symbol(s) not found for architecture x86_64
```

These older clang versions need an explicit implementation definition.

Closes #7286 from xhochy/ARROW-8962

Authored-by: Uwe L. Korn <5f6f0a63dee51498bdd8036770b3bad30ce7a5ac@quantco.com>
Signed-off-by: François Saint-Jacques <76801b7d5fe2c448a8401d2246372be49d372480@gmail.com>

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