[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/xsave.cpp

@@ -0,0 +1,30 @@
+#include <stdint.h>
+#include <cpu/cpu_id.h>
+
+#include "cpu.h"
+#include "xsave.h"
+
+uint64_t xcr0_val = 0;
+static size_t xsave_size_val = 0;
+const size_t &xsave_size = xsave_size_val;
+
+void
+xsave_init()
+{
+    cpu::cpu_id cpuid;
+    const auto regs = cpuid.get(0x0d);
+    const uint64_t cpu_supported = 
+        (static_cast<uint64_t>(regs.edx) << 32) |
+        static_cast<uint64_t>(regs.eax);
+
+    xcr0_val = static_cast<uint64_t>(xcr0::J6_SUPPORTED) & cpu_supported;
+    xsave_size_val = regs.ebx;
+}
+
+void
+xsave_enable()
+{
+    const uint64_t rax = (xcr0_val & 0xFFFFFFFF);
+    const uint64_t rdx = (xcr0_val >> 32);
+    asm volatile ( "xsetbv" :: "c"(0), "d"(xcr0_val >> 32), "a"(xcr0_val) );
+}