[libc] Add explicit constructor calls to fix compilation when using UInt<T>

This patch is similar to 86fe88c8 and adds several explicit
constructor calls (bool(...), uint64_t(...), uint8_t(...)) that are
needed when we use UInt<T> (in my case UInt<128> in riscv32).

This patch also adds two operators to UInt<T>:
* operator/= required by printf_core/float_hex_converter.h:148
* operator-- required by FPUtil/ManipulationFunctions.h:166

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D149594