[go][syscalls] Make zx.Time and zx.Duration signed types

This change is part of a hard transition.

Zircon's C time types, zx_time_t and zx_duration_t, are now signed.
Update Go's to match.

Update TimensecInfinite from UINT64_MAX to INT64_MAX to mirror
ZX_TIME_INFINITE.

Test: CQ; built and ran topaz system on Eve

ZX-2100 #comment

Change-Id: I8e869abefc5496ac6f18fb13cc63845a7da54e47
diff --git a/src/syscall/zx/types.go b/src/syscall/zx/types.go
index f624bc4..47ca623 100644
--- a/src/syscall/zx/types.go
+++ b/src/syscall/zx/types.go
@@ -11,8 +11,8 @@
 type Status int32    // zx_status_t
 type Handle uint32   // zx_handle_t
 type Clock uint32    // zx_clock_t,
-type Time uint64     // zx_time_t, nanoseconds
-type Duration uint64 // zx_duration_t, nanoseconds
+type Time int64      // zx_time_t, nanoseconds
+type Duration int64  // zx_duration_t, nanoseconds
 type Signals uint32  // zx_signals_t
 type Rights uint32   // zx_rights_t
 type Paddr uintptr   // zx_paddr_t
@@ -392,7 +392,7 @@
 )
 
 const (
-	TimensecInfinite = Time(0xFFFFFFFFFFFFFFFF)
+	TimensecInfinite = Time(0x7FFFFFFFFFFFFFFF)
 )
 
 // EOF is used like io.EOF.