Start on support for aarch64

- Add aarch64 support
   - crt0 - add support to _start
   - platform - aarch64 does not have the open syscall. Instead most
     libc implementations use openat() with AT_FDCWD
 - Use sc instead of syscall. sc is the maintained fork of syscall.

platform/src/linux/mod.rs

@@ -1,5 +1,7 @@
 use types::*;
 
+const AT_FDCWD: c_int = -100;
+
 pub fn close(fildes: c_int) -> c_int {
     unsafe {
         syscall!(CLOSE, fildes) as c_int
@@ -13,12 +15,21 @@ pub fn exit(status: c_int) -> ! {
     loop {}
 }
 
+#[cfg(target_arch = "x86_64")]
 pub fn open(path: *const c_char, oflag: c_int, mode: mode_t) -> c_int {
     unsafe {
         syscall!(OPEN, path, oflag, mode) as c_int
     }
 }
 
+#[cfg(target_arch = "aarch64")]
+pub fn open(path: *const c_char, oflag: c_int, mode: mode_t) -> c_int {
+    unsafe {
+        syscall!(OPENAT, AT_FDCWD, path, oflag, mode) as c_int
+    }
+}
+
+
 pub fn write(fildes: c_int, buf: &[u8]) -> ssize_t {
     unsafe {
         syscall!(WRITE, fildes, buf.as_ptr(), buf.len()) as ssize_t