exec/cpu: Rename PAGE_BITS macro to PAGE_RWX

exec/cpu: Rename PAGE_BITS macro to PAGE_RWX

This macro can be used to abbreviate PAGE_READ | PAGE_WRITE | PAGE_EXEC
for which PAGE_RWX is a better name and renaming it also shows it is
not related to TARGET_PAGE_BITS.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240505121008.44A0D4E602D@zero.eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>