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GH-34771: [C++] Add support for compiling on FreeBSD/amd64 (#34772)

### Rationale for this change

addresses issue #34771 

compilation on FreeBSD/amd64 was not working as architecture was not recognised correctly.

### What changes are included in this PR?

minor fix to correct the recognition of the `amd64` architecture.

### Are these changes tested?

test: compilation

### Are there any user-facing changes?

no

* Closes: #34771

Authored-by: Alexander Diemand <codieplusplus@apax.net>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>