exec: Rename exec-vary.c as page-vary.c

exec-vary.c is about variable page size handling,
rename it page-vary.c. Currently this file is target
specific (built once for each target), comment this.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210322112427.4045204-2-f4bug@amsat.org>
[rth: Update MAINTAINERS]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>