aarch64: add focused smoke coverage for multiply-by-constant codegen

[bbbbbq]

Background

We recently fixed an AArch64 codegen regression around multiply-by-constant / move-forwarding patterns. The broader AArch64 suites contain real programs that may hit similar code, but there is no small focused smoke test that directly covers multiply-by-constant correctness.

This is a good first issue because it is mostly test work: the goal is to learn the AArch64 test runner and add a tight regression case without touching register allocation or ABI code.

Scope

• Add a focused AArch64 backend smoke test under the existing AArch64 test structure, for example tests/aarch64_backend_single_source/ or an appropriate tests/run/ smoke directory.
• Cover integer expressions using x * 2, x * 3, x * 4, x * 5, x * 7.
• Include negative inputs so sign handling is exercised.
• Compile with compiler -S and run the linked result under qemu.
• Run both -O0 and -O1 if the local runner supports both modes.
• The test should validate program output or exit code.
• Do not require a specific instruction sequence such as add x, x, lsl #1; correctness is the important part.

Suggested Validation

make test-aarch64-single-source-smoke
# or run the new test script directly

Acceptance Criteria

• The new smoke test fails clearly if generated AArch64 code computes a wrong multiply-by-constant result.
• The test covers at least 2, 3, 4, 5, and 7 multipliers plus negative input values.
• The test passes on the current AArch64 backend.
• No backend implementation changes are required unless the new test exposes a real bug.