mm: smaps: Report arm64 guarded pages in smaps

The arm64 Branch Target Identification support is activated by marking
executable pages as guarded pages.  Report pages mapped this way in
smaps to aid diagnostics.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Daniel Kiss <daniel.kiss@arm.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>