feat(userspace): add ring-3 runtime startup and syscall wrappers

user/crt.S

@@ -0,0 +1,12 @@
+.section .text
+.global _start
+_start:
+    xorq %rbp, %rbp
+    call main
+    movq %rax, %rdi
+    movq $0, %rax
+    int $0x80
+1:
+    jmp 1b
+
+.section .note.GNU-stack, "", @progbits