[kernel] Add MXCSR handling, mask SIMD exceptions

Now threads inherit their MXCSR (sans exception state bits) SIMD
settings from their creator. By default, all exceptions are masked, and
both "to zero" flags are set.

src/kernel/objects/thread.cpp

@@ -34,6 +34,15 @@ thread::thread(process &parent, uint8_t pri, uintptr_t rsp0) :
         m_tcb.rsp0 = rsp0;
 
     m_creator = current_cpu().thread;
+
+    asm volatile ( "stmxcsr %0" : "=m"(m_mxcsr) );
+    m_mxcsr
+        .clear(mxcsr::IE)
+        .clear(mxcsr::DE)
+        .clear(mxcsr::ZE)
+        .clear(mxcsr::OE)
+        .clear(mxcsr::UE)
+        .clear(mxcsr::PE);
 }
 
 thread::~thread()