rvvm_isolation: Fix build on arm32, fix SIGSYS crash with tcc

- ARM32 doesn't define __NR_mmap, only __NR_mmap2 is used
- Cover every other syscall in ifdefs just to be totally sure
- Allow clone3() and a few other new useful syscalls
- Don't use seccomp isolation with non-GNU compilers to prevent crashes on TCC and the likes