Add ET_DYN support for the ELF loader, which required adding

relocations, add some more functions to libxunil, use include inside of
tests instead of manually defining types and add basic libc header
files. Move libxunil to the user folder, and add helloworld and
doomgeneric as apps

user/libxunil/src/syscall.rs

@@ -0,0 +1,32 @@
+#[inline(always)]
+pub unsafe fn syscall0(num: usize) -> isize {
+    let ret: isize;
+    unsafe {
+        core::arch::asm!(
+            "int 0x80",
+            in("rax") num,
+            lateout("rax") ret,
+            options(nostack)
+        );
+    }
+
+    ret
+}
+
+#[inline(always)]
+pub unsafe fn syscall3(num: usize, arg0: usize, arg1: usize, arg2: usize) -> isize {
+    let ret: isize;
+    unsafe {
+        core::arch::asm!(
+            "int 0x80",
+            in("rax") num,
+            in("rdi") arg0,
+            in("rsi") arg1,
+            in("rdx") arg2,
+            lateout("rax") ret,
+            options(nostack)
+        );
+    }
+
+    ret
+}