fix: added arc4random_buf stub

this fixes linking error on aarch64
also _seralPutchar was made public to prevent mangling

Kernel/Source/IO/uart.swift

@@ -1,7 +1,8 @@
 let kUARTBaseAddress: UInt = 0x09000000 // TODO: Make it dynamic by parsion DTB
 
 @_cdecl("putchar")
-@discardableResult func _serialPutchar(_ char: Int32) -> Int32 {
+@discardableResult
+public func _serialPutchar(_ char: Int32) -> Int32 {
     let uartFR: UInt = kUARTBaseAddress + 0x18// TXFF -- TRansmit FIFO Full for PL011 is 5 bit of FR reg (0x18)
 
     while (mmio_read32(uartFR) & (1 << 5)) != 0 {
@@ -16,4 +17,4 @@ public func kprint(_ message: StaticString) {
     for i in 0..<message.utf8CodeUnitCount {
         _serialPutchar(Int32(message.utf8Start[i]))
     }
-}
+}