[kernel] Improve process init

Move process init from each process needing a main.s with _start to
crt0.s in libc. Also change to a sysv-like initial stack with a
j6-specific array of initialization values after the program arguments.

src/libraries/libc/arch/x86_64/crt0.s

@@ -0,0 +1,19 @@
+extern main
+extern exit
+extern _init_libc
+
+global _start:function (_start.end - _start)
+_start:
+	mov rbp, rsp
+	mov rdi, rsp
+
+	call _init_libc
+
+	pop rdi
+	mov rsi, rsp
+
+	call main
+
+	mov rdi, rax
+	call exit
+.end: