Integrate uACPI & remove own ACPI parsing code

This commit adds uACPI as a submodule and removes all handwritten table
parsing code as well as struct definitions, since they now work via
uACPI. There's also now an ability to shut down by using the power
button.

kernel-src/include/uacpi_libc.h

@@ -0,0 +1,11 @@
+#include <string.h>
+#include <printf.h>
+
+#define uacpi_memcpy memcpy
+#define uacpi_memset memset
+#define uacpi_memcmp memcmp
+#define uacpi_strcmp strcmp
+#define uacpi_strlen strlen
+
+#define uacpi_snprintf snprintf
+#define uacpi_vsnprintf vsnprintf