x/sys/unix: move gccgo redeclared functions to separate files

These functions are redeclared in gccgo builds, so move them
to separate files with the "!gccgo" build constraint.

Fixes golang/go#18312

Change-Id: I868cfb77fc394e5e96cee78af0f3abf43db6082a
Reviewed-on: https://go-review.googlesource.com/34379
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_unix.go

@@ -49,11 +49,6 @@ func errnoErr(e syscall.Errno) error {
 	return e
 }
 
-func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)
-func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
-func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err syscall.Errno)
-func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err syscall.Errno)
-
 // Mmap manager, for use by operating system-specific implementations.
 
 type mmapper struct {