Add ET_EXEC ELF loading and validation, and a non-working ET_REL

implementation, running support, also basic syscalls to test if it
works. add a syscall ABI and a new libxunil ABI for calling back to the
kernel with syscalls, add user mode protection when running ELF, add TSS
Privilege Stack table to fix GPF issues, add invalid opcode handler, fix
rust panics using rectangle_filled instead of fb clear. Also add a hello
world example binary that runs in usermode at startup.

libxunil/tests/compile.sh

@@ -0,0 +1,4 @@
+gcc -nostdlib -nostdinc -static -no-pie \
+    -o $1 $1.c \
+    -L../target/release -l:libxunil.a
+mv $1 ../../assets/$1.elf