lif: drop support for go1.5

This change drops a workaround for golang.org/issues/13372.

The compiler in Go 1.6 or above handles the "conversion of a
unsafe.Pointer to uintptr when calling syscall.Syscall" case
correctly.

Change-Id: I5b45094a78f15a631da277bbea0cb79fde25bb2a
Reviewed-on: https://go-review.googlesource.com/37170Reviewed-by: Ian Lance Taylor <iant@golang.org>

lif/sys_solaris_amd64.s

@@ -6,6 +6,3 @@
 
 TEXT ·sysvicall6(SB),NOSPLIT,$0-88
 	JMP	syscall·sysvicall6(SB)
-
-TEXT ·keepAlive(SB),NOSPLIT,$0
-	RET

lif/syscall.go

@@ -19,13 +19,8 @@ var procIoctl uintptr
 
 func sysvicall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (uintptr, uintptr, syscall.Errno)
 
-// TODO: replace with runtime.KeepAlive when available
-//go:noescape
-func keepAlive(p unsafe.Pointer)
-
 func ioctl(s, ioc uintptr, arg unsafe.Pointer) error {
 	_, _, errno := sysvicall6(uintptr(unsafe.Pointer(&procIoctl)), 3, s, ioc, uintptr(arg), 0, 0, 0)
-	keepAlive(arg)
 	if errno != 0 {
 		return error(errno)
 	}