Make the Amount constructor noexcept

Summary: Because it can't throw but that's not implied by constexpr. This fixes a build issue with clang8 that requires a noexcept constructor for the atomic, so we need that noexcept to build a std::atomic<Amount> which we do in net.h.

Test Plan:
With clang 8:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13803