[libc] Zero initialize Uint<T>::val

This fix a regression introduced by commit 5db12eca

Fixes the compilation failure reported by libc-x86_64-debian-gcc-fullbuild-dbg:

.../libc/src/__support/UInt.h:26:31: note: ‘struct __llvm_libc::cpp::UInt<192>’ has no user-provided default constructor
   26 | template <size_t Bits> struct UInt {
      |                               ^~~~
.../libc/src/__support/UInt.h:38:13: note: constructor is not user-provided because it is explicitly defaulted in the class body
   38 |   constexpr UInt() = default;