linux-user: fails to deliver signals raised during pselect
Description of problem:
When run via qemu a program which blocks signals but unmasks them during `pselect` does not catch these signals when returning from `pselect`.

Used as reference on expected behavior: [The new pselect() system call](https://lwn.net/Articles/176911/)
Steps to reproduce:
A minimal test case below mimics behavior as encountered in the test suite of `p11-kit` ([link](https://github.com/p11-glue/p11-kit)) (which attempts to catch `SIGTERM` in a similar way and results in lingering processes after running the test suite).

```C
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/select.h>

static void handler(int sig)
{
	puts("SIGNAL");
}

int main(int argc, char *argv[])
{
	struct sigaction sa;

	fd_set rfds;
	sigset_t emptyset, blockset;

	sigemptyset (&blockset);
	sigemptyset (&emptyset);
	sigaddset (&blockset, SIGUSR1);

	sa.sa_handler = handler;
	sigemptyset(&sa.sa_mask);
	sa.sa_flags = 0;
	sigaction(SIGUSR1, &sa, NULL);

	sigprocmask (SIG_BLOCK, &blockset, NULL);

	FD_ZERO(&rfds);

	while(1) {
		pselect(0, &rfds, NULL, NULL, NULL, &emptyset);
	}

	return 0;
}
```

Running this without qemu should print _SIGNAL_ when sent `SIGUSR1`:

```
$ ./a.out &
[1] 1683587
$ kill -USR1 %1
$ SIGNAL
```

When run with `qemu-x86_64` however, it does not (also qemu's `-strace` confirms the signal isn't received whereas a strace of qemu shows it's in fact delivered).

The pselect call itself _is_ interrupted, but the signal goes missing.