Traditionally, MIPS use 4KB page size, but Loongson prefer 16KB page
size in system emulator. So, let's define TARGET_PAGE_BITS_VARY and
TARGET_PAGE_BITS_MIN to support variable page size.
Backports commit ee3863b9d414f0b4a59a88f2a79b496a99d4f6dd from qemu
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.
Backports commit 74433bf083b0766aba81534f92de13194f23ff3e from qemu
