The SET_ARG() macro returns an error indication; we check this in the
TARGET_SYS_GET_CMDLINE case but not when we use it in implementing
TARGET_SYS_ELAPSED.  Check for and handle the errors via the do_fault
codepath, and update the comment documenting the SET_ARG() and
GET_ARG() macros to note how they handle memory access errors.

Resolves: Coverity CID 1490287
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220719121110.225657-4-peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220725140520.515340-9-alex.bennee@linaro.org>