benchtests: Add UNSUPPORTED benchmark status

The libmvec benchmarks print a message indicating that a certain CPU
feature is unsupported and exit prematurelyi, which breaks the JSON in
bench.out.

Handle this more elegantly in the bench makefile target by adding
support for an UNSUPPORTED exit status (77) so that bench.out continues
to have output for valid tests.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>