[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/libraries/util/util/bitset.h

@@ -66,8 +66,6 @@ namespace {
 template <>
 class bitset<64>
 {
-    static constexpr unsigned num_elems = 1;
-
     template <typename T>
     static constexpr uint64_t bit_or(T b) { return 1ull << uint64_t(b); }
 
@@ -127,8 +125,6 @@ private:
 template <>
 class bitset<32>
 {
-    static constexpr unsigned num_elems = 1;
-
     template <typename T>
     static constexpr uint32_t bit_or(T b) { return 1u << uint32_t(b); }