exec: Extract 'page-vary.h' header

exec: Extract 'page-vary.h' header

In the next commit we will extract the generic code out of
page-vary.c, only keeping the target specific code. Both
files will use the same TargetPageBits structure, so make
its declaration in a shared header.

As the common header can not use target specific types,
use a uint64_t to hold the page mask value, and add a
cast back to target_long in the TARGET_PAGE_MASK definitions.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210322112427.4045204-3-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>