[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:

src/libraries/libc/arch/x86_64/init_libc.c

@@ -0,0 +1,36 @@
+#include <stdint.h>
+#include <j6/init.h>
+#include <j6/types.h>
+
+static size_t __initc = 0;
+static struct j6_init_value *__initv = 0;
+
+j6_handle_t __handle_sys = j6_handle_invalid;
+
+void
+_get_init(size_t *initc, struct j6_init_value **initv)
+{
+	if (!initc)
+		return;
+
+	*initc = __initc;
+	if (initv)
+		*initv = __initv;
+}
+
+void
+_init_libc(uint64_t *rsp)
+{
+	uint64_t argc = *rsp++;
+	rsp += argc;
+
+	__initc = *rsp++;
+	__initv = (struct j6_init_value *)rsp;
+
+	for (unsigned i = 0; i < __initc; ++i) {
+		if (__initv[i].type == j6_init_handle_system) {
+			__handle_sys = __initv[i].value;
+			break;
+		}
+	}
+}