The A64 sequence table is populated by registration statics spread across
multiple translation units. Keeping the backing map as a global object makes
registration depend on cross-TU initialization order and can hit the table
before it has been constructed.
Move the sequence table behind a function-local static so sequence
registration is safe regardless of which unit initializes first.
Based entirely off existing xbyak x86 implementation and available
tests. Still needs a lot of optimization and testing on non-Windows
platforms.
So far passes all tests and boots at least some games on Windows.