[kernel] Initial XSAVE support implementation

Initial support for XSAVE, but not XSAVEOPT or XSAVEC:

- Enable XSAVE and set up xcr0 for all CPUs
- Allocate XSAVE area for all non-kernel threads
- Call XSAVE and XRSTOR on task switch

src/kernel/kernel.module

@@ -65,6 +65,7 @@ kernel = module("kernel",
         "tss.cpp",
         "vm_space.cpp",
         "wait_queue.cpp",
+        "xsave.cpp",
     ])
 
 if config == "debug":