This fixes a problem I was having where using frame advance with the
debugger open would frequently cause panic alerts about invalid addresses
due to the CPU thread changing MSR.DR while the host thread was trying
to access memory.
To aid in tracking down all the places where we weren't properly locking
the CPU, I've created a new type (in Core.h) that you have to pass as a
reference or pointer to functions that require running as the CPU thread.
This string is extremely likely to be mistranslated without the
proper context. Actually, it's probably impossible to translate
this string in a good way to some languages, but I'm not sure how
to solve that. Let's at least add an i18 comment for now.
