powerpc/syscall: Fix syscall skip handling for seccomp and ptrace

In the Linux kernel, the following vulnerability has been resolved:

powerpc/syscall: Fix syscall skip handling for seccomp and ptrace

After enabling GENERIC_ENTRY on PowerPC, syscall_enter_from_user_mode()
returns -1 as a sentinel to signal that seccomp or ptrace has intercepted
the syscall and already set a return value via syscall_set_return_value().
system_call_exception() was not handling this sentinel, and since -1UL
is >= NR_syscalls, the code fell into the out-of-range path and returned
-ENOSYS, overwriting the errno already placed in regs->gpr[3].

The naive fix of checking r0 == -1L before the NR_syscalls bounds check
is ambiguous: a user legitimately calling syscall(-1) also produces r0 ==
-1L, and a tracer intercepting such a call would have its injected return
value silently discarded.

Fix this by introducing a thread flag that is set whenever
syscall_set_return_value() explicitly updates the return value. In
system_call_exception(), check and clear this flag before dispatching
the syscall, and return the preset value directly when it is present.
This ensures that an explicitly supplied return value always suppresses
syscall execution, regardless of the syscall number.

This handles all seccomp actions correctly:

- SECCOMP_RET_ERRNO, SECCOMP_RET_TRACE (no tracer), SECCOMP_RET_USER_NOTIF:
all call syscall_set_return_value(), flag is set, injected value returned.
- SECCOMP_RET_TRAP, SECCOMP_RET_KILL: call syscall_rollback() and deliver
a signal; flag is not set, but the process is dying so the return value
is irrelevant.

The fix covers both ppc32 and ppc64 with no #ifdefs.