Fix NO_VM_COMPILED CMake wiring for unsupported CPU builds

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Summary

Corrects the QVM JIT vs interpreter build selection in the top-level CMakeLists.txt. The previous ELSE() branch called target_compile_definitions on qcommon / qcommon_ded before those targets were created (invalid CMake), and the follow-up IF(NO_VM_COMPILED) never ran because NO_VM_COMPILED was never set.

Flags removed / gates simplified

• Build-time NO_VM_COMPILED gate: The dead/invalid path that tried to apply -DNO_VM_COMPILED in the CPU ELSE() before add_library(qcommon …) is removed. The macro is now applied only after qcommon / qcommon_ded exist, when no JIT backend source is selected.

Why this is safe to change

• Supported architectures (x86/x86_64, aarch64, arm*, ppc64*) still append the same vm_*.c files as before; NO_VM_COMPILED stays off, matching prior intent.
• Unsupported / novel CPUs now reliably get NO_VM_COMPILED=1 instead of silently building without the define (which could mismatch vm.c expectations vs. accidentally linked JIT objects).

Behavioral parity checks

• cmake -B … -S . -G Ninja on native x86_64: configures successfully; cmake --build … --target qcommon qcommon_ded succeeds.
• Fresh configure with -DCMAKE_SYSTEM_NAME=Linux -DCMAKE_SYSTEM_PROCESSOR=riscv64: build.ninja shows -DNO_VM_COMPILED=1 on qcommon sources and no vm_x86.c (etc.) objects; cmake --build … --target qcommon succeeds.

Notes

• This repository does not use web-style “feature flag” services; engine toggles are primarily cvars and compile-time defines. Runtime cvars such as r_ext_* remain user-tunable and were not removed.