build: default to a 64-bit only build for the Capstone decoder

[lvaroqui]

Default to a 64-bit only build, and fail at configure time if a secondary platform is still requested.

On a multilib host (Arch with lib32-glibc, Ubuntu with gcc-multilib), a plain ./configure also enables the x86 secondary platform. Capstone is only compiled for the primary platform, and callgrind/Makefile.am passes the Capstone flags only to the primary targets, so callgrind_x86_linux failed in make on the #error in cycledecode.c:350. CI and the Debian packaging never hit this: they pass --enable-only64bit, and the Ubuntu runners have no 32-bit toolchain.

CODSPEED_ONLY64BIT_DEFAULT sets --enable-only64bit unless --enable-only64bit or --enable-only32bit was given either way. CODSPEED_CAPSTONE now errors out when a secondary platform remains (e.g. --disable-only64bit), instead of failing later in make.

Alternatives considered:

• Building a second, -m32 Capstone and wiring it into the secondary targets: nothing runs a 32-bit Callgrind with cycle estimation, so it isn't worth the extra build.
• Stubbing cycledecode.c when Capstone is absent: this would silently ship a Callgrind whose --cycle-estimation does nothing.

Verified on a multilib Manjaro host: plain ./configure gives amd64 only and make succeeds; --disable-only64bit stops at configure with the new message; --enable-only32bit configures an x86-only build.

Closes COD-3590

[codspeed]

Merging this PR will not alter performance

✅ 84 untouched benchmarks
⏩ 60 skipped benchmarks¹

---

_{Comparing cod-3590-valgrind-codspeed-fails-to-build-on-multilib-hosts-no (5429ea2) with master (18b901a)}

Footnotes

1. 60 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[greptile-apps]

Confidence Score: 5/5

[High risk] Changes build configuration defaults and constraints.

The PR appears safe to merge based on the reviewed changes.

Summary

The PR defaults configuration to a 64-bit-only build and rejects secondary-platform builds before Capstone-dependent Callgrind compilation begins. The latest change sets the 64-bit cache value directly so a cached bi-architecture choice does not defeat that default.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Configure options and cache] --> B[Choose primary and secondary platforms]
  B --> C{Secondary platform selected?}
  C -->|Yes| D[Stop at Capstone configuration check]
  C -->|No| E[Build primary-platform Callgrind]

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