exec: Split out variable page size support to exec-vary.c

exec: Split out variable page size support to exec-vary.c

The next patch will play a trick with "const" that will
confuse the compiler about the uses of target_page_bits
within exec.c.  Moving everything to a new file prevents
this confusion.

No functional change so far.

Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>