parisc: Fix double SIGFPE crash

Camm noticed that on parisc a SIGFPE exception will crash an application with
a second SIGFPE in the signal handler.  Dave analyzed it, and it happens
because glibc uses a double-word floating-point store to atomically update
function descriptors. As a result of lazy binding, we hit a floating-point
store in fpe_func almost immediately.

When the T bit is set, an assist exception trap occurs when when the
co-processor encounters *any* floating-point instruction except for a double
store of register %fr0.  The latter cancels all pending traps.  Let's fix this
by clearing the Trap (T) bit in the FP status register before returning to the
signal handler in userspace.

The issue can be reproduced with this test program:

root@parisc:~# cat fpe.c

static void fpe_func(int sig, siginfo_t *i, void *v) {
        sigset_t set;
        sigemptyset(&set);
        sigaddset(&set, SIGFPE);
        sigprocmask(SIG_UNBLOCK, &set, NULL);
        printf("GOT signal %d with si_code %ld\n", sig, i->si_code);
}

int main() {
        struct sigaction action = {
                .sa_sigaction = fpe_func,
                .sa_flags = SA_RESTART|SA_SIGINFO };
        sigaction(SIGFPE, &action, 0);
        feenableexcept(FE_OVERFLOW);
        return printf("%lf\n",1.7976931348623158E308*1.7976931348623158E308);
}

root@parisc:~# gcc fpe.c -lm
root@parisc:~# ./a.out
 Floating point exception

root@parisc:~# strace -f ./a.out
 execve("./a.out", ["./a.out"], 0xf9ac7034 /* 20 vars */) = 0
 getrlimit(RLIMIT_STACK, {rlim_cur=8192*1024, rlim_max=RLIM_INFINITY}) = 0
 ...
 rt_sigaction(SIGFPE, {sa_handler=0x1110a, sa_mask=[], sa_flags=SA_RESTART|SA_SIGINFO}, NULL, 8) = 0
 --- SIGFPE {si_signo=SIGFPE, si_code=FPE_FLTOVF, si_addr=0x1078f} ---
 --- SIGFPE {si_signo=SIGFPE, si_code=FPE_FLTOVF, si_addr=0xf8f21237} ---
 +++ killed by SIGFPE +++
 Floating point exception

Signed-off-by: Helge Deller <deller@gmx.de>
Suggested-by: John David Anglin <dave.anglin@bell.net>
Reported-by: Camm Maguire <camm@maguirefamily.org>
Cc: stable@vger.kernel.org