linux-user/strace: show TID instead of PID

This aligns with strace, and is very useful when tracing multi-threaded
programs. The result is the same in single-threaded programs.

gettid() requires the _GNU_SOURCE feature test macro, so it might be
unavailable in rare cases. I don't expect it to be a problem though,
because it's implemented by both glibc and musl.

Signed-off-by: J. Neuschäfer <[email protected]>

linux-user/strace.c

@@ -4337,7 +4337,7 @@ print_syscall(CPUArchState *cpu_env, int num,
     if (!f) {
         return;
     }
-    fprintf(f, "%d ", getpid());
+    fprintf(f, "%d ", gettid());
 
     for (i = 0; i < nsyscalls; i++) {
         if (scnames[i].nr == num) {