linux-user/strace: Expand struct flags to hold a mask

A zero bit value does not make sense -- it must relate to
some field in some way.

Define FLAG_BASIC with a build-time sanity check.
Adjust FLAG_GENERIC and FLAG_TARGET to use it.
Add FLAG_GENERIC_MASK and FLAG_TARGET_MASK.

Fix up the existing flag definitions for build errors.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230707204054.8792-6-richard.henderson@linaro.org>