hppa: workaround kernel bug in syscall number tampering

hppa: workaround kernel bug in syscall number tampering

Linux kernel commit v4.6-rc2~20^2 introduced a regression:
when tracer changes syscall number to -1, the kernel fails
to initialize %r28 with -ENOSYS and subsequently fails
to return the error code of the failed syscall to userspace.
Workaround this by initializing %r28 ourselves.

* linux/arch_defs_.h (ARCH_NEEDS_SET_ERROR_FOR_SCNO_TAMPERING): Define
to 0.
* linux/hppa/arch_defs_.h (ARCH_NEEDS_SET_ERROR_FOR_SCNO_TAMPERING):
Define to 1.
* syscall.c (tamper_with_syscall_entering)
[ARCH_NEEDS_SET_ERROR_FOR_SCNO_TAMPERING]: When changing syscall number
to -1, set the return value as well.