For all targets, into this new file move TARGET_LONG_BITS,
TARGET_PAGE_BITS, TARGET_PHYS_ADDR_SPACE_BITS,
TARGET_VIRT_ADDR_SPACE_BITS, and NB_MMU_MODES.

Include this new file from exec/cpu-defs.h.

This now removes the somewhat odd requirement that target/arch/cpu.h
defines TARGET_LONG_BITS before including exec/cpu-defs.h, so push the
bulk of the includes within target/arch/cpu.h to the top.
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Acked-by: NAlistair Francis <alistair.francis@wdc.com>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>