alpha, ia64, mips, nios2, powerpc, sparc: always check for syscall error

alpha, ia64, mips, nios2, powerpc, sparc: always check for syscall error

On architectures that use dedicated registers to report syscall errors,
check for syscall error condition regardless of SYSCALL_NEVER_FAILS flag
in the syscall entry.  On architectures that use negated errno
semantics, there is no way to distinguish a syscall error from a valid
return code that looks like a syscall error, and the check for
SYSCALL_NEVER_FAILS flag remains the only way to tell one case
from another.

* linux/alpha/get_error.c (get_error): Ignore check_errno.
* linux/ia64/get_error.c: Likewise.
* linux/mips/get_error.c: Likewise.
* linux/nios2/get_error.c: Likewise.
* linux/powerpc/get_error.c: Likewise.
* linux/sparc/get_error.c: Likewise.
* linux/sparc64/get_error.c: Likewise.