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/lib.rs

@@ -0,0 +1,67 @@
+#![no_std]
+#![no_main]
+
+use crate::syscall::syscall3;
+
+pub mod syscall;
+
+const SYS_EXIT: usize = 1;
+const SYS_WRITE: usize = 60;
+
+#[unsafe(no_mangle)]
+extern "C" fn write(fd: i32, buf: *const u8, count: usize) -> isize {
+    unsafe { syscall3(SYS_WRITE, fd as usize, buf as usize, count) }
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn exit(code: i32) -> ! {
+    unsafe { syscall3(SYS_EXIT, code as usize, 0, 0) };
+    loop {}
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn strlen(s: *const u8) -> usize {
+    let mut len = 0;
+    while unsafe { *s.add(len) } != 0 {
+        len += 1;
+    }
+    len
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn puts(s: *const u8) -> isize {
+    write(1, s, strlen(s));
+
+    0
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn abs(n: i32) -> i32 {
+    n.abs()
+}
+
+#[unsafe(no_mangle)]
+extern "C" fn atoi(mut c: *const u8) -> i32 {
+    let mut value: i32 = 0;
+    let mut sign: i32 = 1;
+    unsafe {
+        if (*c) == b'+' || (*c) == b'-' {
+            if *c == b'-' {
+                sign = -1;
+            }
+            c = c.add(1);
+        }
+        while (*c).is_ascii_digit() {
+            value *= 10;
+            value += ((*c) - b'0') as i32;
+            c = c.add(1);
+        }
+    }
+
+    value * sign
+}
+
+#[panic_handler]
+fn panic(_: &core::panic::PanicInfo) -> ! {
+    loop {}
+}