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GH-43688: [C++] Prevent Snappy from disabling RTTI when bundled (#43706)

### Rationale for this change

Snappy's CMakeLists.txt unconditionally disables RTTI. This is incompatible with some other options, such as activating UBSAN for a fuzzing build:
https://github.com/google/snappy/issues/189

### What changes are included in this PR?

Add `-frtti` at the end of compiler options when compiling a bundled Snappy build.

### Are these changes tested?

On CI; also manually checked that this allows enabling Snappy on OSS-Fuzz builds.

### Are there any user-facing changes?

No.

* GitHub Issue: #43688

Lead-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>