Revived GUEST_BASE support for usermode emulation targets [v5]

Revived GUEST_BASE support for usermode emulation targets [v5]

- Now GUEST_BASE is dynamic and can be set from command line.
- Qemu checks /proc/sys/vm/mmap_min_addr and sets GUEST_BASE
  if needed.
- Code generation supports GUEST_BASE for i386 and x86_64 hosts.

[v3]: implemented GUEST_BASE with offset mods

Using malc's advice, instead of a mov or lea, just adjust
the offset parameter of tcg_out_modrm_offset(). - Riku

[v5]: set a GUEST_BASE_MAX instead of supporting moving
to arbitary ranges

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>