GH-50631: [C++] Scope macOS SDK 11.3 simdjson workaround to bundled simdjson

[Reranko05]

Rationale for this change

The workaround introduced for the macOS 11.3 SDK currently uses a global add_compile_definitions(SIMDJSON_CONCEPT_DISABLED=1).

Since this workaround is only needed when building the bundled simdjson dependency, scope it to the simdjson target inside build_simdjson() instead of applying it globally.

What changes are included in this PR?

• Move the macOS SDK 11.3 workaround into build_simdjson().
• Replace the global add_compile_definitions() with target_compile_definitions(simdjson PRIVATE SIMDJSON_CONCEPT_DISABLED=1).

• GitHub Issue: [C++] Scope macOS SDK 11.3 simdjson workaround to build_simdjson #50631

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[github-actions]

⚠️ GitHub issue #50631 has been automatically assigned in GitHub to PR creator.

[Reranko05]

@kou As suggested, I opened an upstream issue for this: simdjson/simdjson#2787

[kou]

@github-actions crossbow submit -g r

[github-actions]

Revision: fa60e5d

Submitted crossbow builds: ursacomputing/crossbow @ actions-343cdd513e

Task	Status
r-binary-packages
r-recheck-most
test-r-alpine-linux-cran
test-r-arrow-backwards-compatibility
test-r-depsource-system
test-r-dev-duckdb
test-r-devdocs
test-r-extra-packages
test-r-fedora-clang
test-r-gcc-11
test-r-gcc-12
test-r-install-local
test-r-install-local-minsizerel
test-r-linux-as-cran
test-r-linux-rchk
test-r-linux-sanitizers
test-r-linux-valgrind
test-r-m1-san
test-r-macos-as-cran
test-r-offline-maximal
test-r-ubuntu-22.04
test-r-versions
test-r-wasm

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