x86: detect Linux userspace ISA extensions

[nihui]

Expand x86 userspace ISA detection based on features currently recognized by upstream Linux/KVM, while keeping ruapu's existing execute-and-trap style.

This revision intentionally keeps the scope conservative: probes are implemented as direct RUAPU_INSTCODE() byte sequences only. Features that require scratch-buffer wrappers, ABI-specific helper functions, process-state changes, special devices, unusual naming special cases, overlapping encodings that cannot uniquely identify the Linux feature, or have ambiguous exception/compatibility semantics are deferred.

New probes include:

• legacy/vendor: cmov, lahflm, tbm, rdpru
• cache/data movement: clflush, clflushopt, clwb, movbe, movrs (x86-64 only)
• scalar/general: pclmulqdq, fsgsbase (x86-64 only), adx, pku, rdpid, serialize
• vector: avx512pf, avx512bitalg, avx512vpopcntdq, avx512vp2intersect, avx512bmm, vpclmulqdq
• x86-64: apx

The previous helper-heavy probes for cx8/cx16, xsave*, movdiri/movdir64b, clzero, cmpccxadd, newer AMX subsets, and VIA PadLock are removed from this PR for now. Likewise, probes with questionable execute-and-trap semantics or platform-dependent faults such as cldemote, xgetbv1, waitpkg, tsxldtrk, rtm, tsc, and rdtscp are intentionally deferred.

mmxext is also deferred because the selected PAVGB MMX encoding is shared with SSE1-capable processors and therefore cannot uniquely represent Linux X86_FEATURE_MMXEXT under ruapu's execute-and-trap model. 3dnow and 3dnowext are deferred because their public names are not valid C identifiers and would require special-case internal naming/mapping.

The x86 probe list is organized by ISA family, and README.md, main.c, and g_ruapu_isa_map are kept in sync. The branch is squashed to one commit.