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_linux_amd64.go

@@ -6,8 +6,6 @@
 
 package unix
 
-import "syscall"
-
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	EpollWait(epfd int, events []EpollEvent, msec int) (n int, err error)
 //sys	Fadvise(fd int, offset int64, length int64, advice int) (err error) = SYS_FADVISE64
@@ -63,9 +61,6 @@ import "syscall"
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)
 
-//go:noescape
-func gettimeofday(tv *Timeval) (err syscall.Errno)
-
 func Gettimeofday(tv *Timeval) (err error) {
 	errno := gettimeofday(tv)
 	if errno != 0 {