thread safety

[bchurchill]

While generally x64asm need not offer thread safety (consumers of the library are responsible for creating per-thread copies), there are a few places where it makes use of global variables without any kind of locking/mutex.

• in the constructor Label::Label
• in the function that computes the hex_size of an instruction

This issue is to fix these so that using x64asm in a multithreaded environment is possible.