unix: remove raw syscall from Getfsstat

Use libc version instead.

Update golang/go#17490

Change-Id: Ibcedccb9715961904d12fc85f69a7139b6c26658
Reviewed-on: https://go-review.googlesource.com/c/154660
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/darwin_test.go

@@ -206,4 +206,5 @@ var darwinTests = [...]darwinTest{
 	{"mmap", libc_mmap_trampoline},
 	{"munmap", libc_munmap_trampoline},
 	{"gettimeofday", libc_gettimeofday_trampoline},
+	{"getfsstat64", libc_getfsstat64_trampoline},
 }