Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

kernel/arch/x86_64/swtch.S

@@ -0,0 +1,30 @@
+# Context switch
+#
+#   void swtch(struct context **old, struct context *new);
+#
+# Save current register context in old
+# and then load register context from new.
+
+.global swtch
+swtch:
+  # Save old callee-save registers
+  pushq   %rbp
+  pushq   %rbx
+  pushq   %r12
+  pushq   %r13
+  pushq   %r14
+  pushq   %r15
+
+  # Switch stacks
+  movq    %rsp, (%rdi)
+  movq    %rsi, %rsp
+
+  # Load new callee-save registers
+  popq    %r15
+  popq    %r14
+  popq    %r13
+  popq    %r12
+  popq    %rbx
+  popq    %rbp
+
+  retq #??