kern: initramfs: add missing uart_amd64.v and uart_arm64.v

Fixes CI failure: initramfs.v references uart_putc, but the
platform-specific definitions were never committed.

Author: medvednikov

kernel/modules/initramfs/uart_amd64.v

@@ -0,0 +1,7 @@
+module initramfs
+
+import dev.serial
+
+fn uart_putc(c u8) {
+	serial.out(c)
+}

kernel/modules/initramfs/uart_arm64.v

@@ -0,0 +1,7 @@
+module initramfs
+
+fn C.aarch64__uart__putc(c u8)
+
+fn uart_putc(c u8) {
+	C.aarch64__uart__putc(c)
+}