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: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>