feat: add process termination codes and SYS_ABORT; whole-process abort() from any thread in libc and montauk-rt

programs/include/montauk/syscall.h

@@ -527,6 +527,11 @@ namespace montauk {
     inline int kill(int pid) {
         return (int)syscall1(montauk::abi::SYS_KILL, (uint64_t)pid);
     }
+    // End the whole process from any thread (exit code TerminatedAborted).
+    [[noreturn]] inline void abort() {
+        syscall0(montauk::abi::SYS_ABORT);
+        __builtin_unreachable();
+    }
     inline int setsession() {
         return (int)syscall0(montauk::abi::SYS_SETSESSION);
     }