rvvm_user: standalone userland runner with Linux syscall layer hardening

[linsmod]

Motivation

Upstream ships rvvm_user.c as dormant experimental code: RVVM_USER_TEST is
commented out and there is no build target for it. This PR makes the userland
runner actually usable as a standalone rvvm_user binary, and hardens the
Linux syscall layer until real-world static RISC-V binaries run to completion.

For validation this PR borrows guest ELFs from a reproduction package aimed at
user-mode host implementations of a guest-JIT bug: a statically linked
riscv64 musl JS runtime ("ant", cross-compiled with zig) plus a 1MB JS
workload. They are used here solely to verify that rvvm_user runs
real-world static binaries.

Summary

A standalone userland runner (rvvm_user) plus a set of Linux syscall-layer
fixes discovered while running real-world static RISC-V binaries under
user-mode emulation.

Changes

New entry point / build

• src/rvvm_user_main.c: standalone rvvm_user binary entry
• project.mk: builds rvvm_user and defines RVVM_USER_TEST only on
  64-bit Linux targets; all other targets (including i386) keep the existing
  upstream stub behavior unchanged

Syscall correctness fixes

• 425 io_uring_setup: return -ENOSYS instead of falling through
• 123 sched_getaffinity: pass through the host affinity mask
• 21/22 epoll_ctl / epoll_pwait: translate between the packed x86-64 host
  struct epoll_event (12 bytes) and the naturally-aligned guest layout
  (16 bytes); previously events were corrupted on x86-64 hosts
• 259 riscv_flush_icache: optional RVVM_JITDUMP=1 dumps each flushed
  guest code range (useful for debugging guest JITs)
• 220 clone: RVVM_USER_NO_THREADS=1 knob forcing -EAGAIN (debug aid)

Usability

• RVVM_USER_PREFIX env var overrides the hardcoded prefix_path (empty
  value disables the prefix); the hardcoded default silently broke absolute
  guest paths
• RVVM_NOJIT env var for pure-interpreter userland (bisecting JIT issues)
• ELF load failure diagnostics: open retries, distinct open vs VMA-collision
  errors, /proc/self/maps dump

ELF loading

• Map 256MB (ELF_USERLAND_HEAP_MARGIN) past the fixed ELF image as the
  initial brk area, matching how real kernels map pages after the image;
  without this, early brk accesses on non-relocatable static ELFs fault

CI / build hygiene

• prefix_path const-correctness and getcwd/chdir return value checks
  (-Werror clean)
• Silenced GCC -fanalyzer fd-leak false positives: guest fd numbers passed
  through the syscall dispatch are not host descriptors (scoped #pragma,
  GCC 12+ only)

Testing

• make CFLAGS="-Werror" clean on x86_64 GCC/clang, riscv64 cross-compile,
  and a mingw cross-build simulation; make test (riscv-tests) passes
• Smoke-tested with a static riscv64 hello-world (exit code pass-through OK)
• Validated with the borrowed ELF payload with a 1MB JS bundle:
  • https://github.com/linsmod/ant_agent_repro/releases/download/binaries-v1/ant_jit_runner_repro.zip