[kernel] Add userspace threading

Implement the syscalls necessary for threads to create other threads in
their same process. This involved rearranging a number of syscalls, as
well as implementing object_wait and a basic implementation of a
process' list of handles.

src/include/kernel_memory.h

@@ -16,9 +16,6 @@ namespace memory {
 	/// Offset from physical where page tables are mapped.
 	constexpr uintptr_t page_offset = 0xffffc00000000000;
 
-	/// Initial process thread's stack address
-	constexpr uintptr_t initial_stack = 0x0000800000000000;
-
 	/// Initial process thread's stack size, in pages
 	constexpr unsigned initial_stack_pages = 1;