interrupt: fix race condition with errno

Caught by vchuravy!

It's likely that only the jl_safe_printf code to print
"force throwing a sigint" would alter errno anyways
(and thus, this fix may not be observable in practice),
but best to avoid altering `errno` in a signal handler regardless.

src/signals-unix.c

@@ -358,6 +358,7 @@ static void jl_exit_thread0(int state)
 void usr2_handler(int sig, siginfo_t *info, void *ctx)
 {
  jl_ptls_t ptls = jl_get_ptls_states();
+ int errno_save = errno;
  sig_atomic_t request = jl_atomic_exchange(&ptls->signal_request, 0);
 #if !defined(JL_DISABLE_LIBUNWIND)
  if (request == 1) {
@@ -388,6 +389,7 @@ void usr2_handler(int sig, siginfo_t *info, void *ctx)
  else if (request == 3) {
  jl_call_in_ctx(ptls, jl_exit_thread0_cb, sig, ctx);
  }
+ errno = errno_save;
 }
 
 #if defined(HAVE_TIMER)