Base64::encode/decode is not standard base64 enconding

### Describe the bug

The[ `Base64::encode`/`decode` ](https://github.com/intel/llvm/blob/67fa0848e4e67ae2b93d2008328f97da35aa9574/llvm/include/llvm/Support/Base64.h#L64) methods, which are used to encode binary data in the `.prop` files used by `sycl-post-link`, is not the standard base64 encoding (RFC 4648): `Base64::encode("hello world", ...)` gives `oVGbs9GI39mcsRG` while standard base64 encodes this as `aGVsbG8gd29ybGQ=`. The pair of methods do not seem to be flawed in the case of a being able to encode/decode successfully, but this makes it awkward and confusing to make new `sycl-post-link` tests checking the output of the `.prop` files, as they use a nonstandard encoding.

### To reproduce

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### Environment

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### Additional context

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