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 <alex.bennee@linaro.org> Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>