pkg/syscall: Plan 9, 64-bit: Update error checks from sys calls.

The system calls return 32-bit integers. With the recent change
in size of `int' in Go for amd64, the type conversion was not
catching `-1' return values. This change makes the conversion
explicitly `int32'.

R=rsc, rminnich, npe, r
CC=golang-dev
https://golang.org/cl/6576057