i386: move TCG cpu class initialization to tcg/

i386: move TCG cpu class initialization to tcg/

to do this, we need to take code out of cpu.c and helper.c,
and also move some prototypes from cpu.h, for code that is
needed in tcg/xxx_helper.c, and which in turn is part of the
callbacks registered by the class initialization.

Therefore, do some shuffling of the parts of cpu.h that
are only relevant for tcg/, and put them in tcg/helper-tcg.h

For FT0 and similar macros, put them in tcg/fpu-helper.c
since they are used only there.

Signed-off-by: Claudio Fontana <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-Id: <20201212155530[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>