gas: put emul decls in emul.h

gas: put emul decls in emul.h

The individual struct emulation instances shouldn't be declared in a .c
file; it and the producers of the symbols want to both see the
declarations, so declarations and definitions don't go out of sync. Move
these declarations to emul.h.

While there also adjust the conditional around this_format: That symbol
is never #define-d anywhere, and it's needed only when USE_EMULATIONS is
defined. (Really, when obj-multi isn't in use, it also is effectively
only ever written to.)