Kernel: Rewrite whole scheduler

Current context saving was very hacky and dependant on compiler
behaviour that was not consistent. Now we always use iret for
context saving. This makes everything more clean.

libc/arch/x86_64/crt0.S

@@ -2,34 +2,29 @@
 
 .global _start
 _start:
-	# Set up end of the stack frame linked list.
-	movq $0, %rbp
-	pushq %rbp # rip=0
-	pushq %rbp # rbp=0
-	movq %rsp, %rbp
+	# STACK LAYOUT
+	#   null
+	#   argc
+	#   argv
+	#   envp
 
-	# Save argc, argv, environ
-	pushq %rdx
-	pushq %rsi
-	pushq %rdi
+	xorq %rbp, %rbp
 
-	# Prepare malloc, environment
-	movq %rdx, %rdi
+	# init libc
+	movq 0(%rsp), %rdi
 	call _init_libc
 
-	# Call global constructos
+	# call global constructors
 	call _init
 
-	# Restore argc, argv, environ
-	popq %rdi
-	popq %rsi
-	popq %rdx
-
-	# Run main
+	# call main
+	movq 16(%rsp), %rdi
+	movq  8(%rsp), %rsi
+	movq  0(%rsp), %rdx
 	call main
 
-	# Cleanly exit the process
-	movl %eax, %edi
+	# call exit
+	movq %rax, %rdi
 	call exit
 
 .size _start, . - _start