properly check valgrind supported architectures
    
    libinsane assumes valgrind is available on all arches,
    but for riscv there is still work in progress and riscv64 is
    not yet supported, thus resulting in the following
    `No matching package to install: 'valgrind'`
    error. Also there is ongoing work for adding support for riscv64
    https://bugs.kde.org/show_bug.cgi?id=468575
    
    Signed-off-by: Kanak Shilledar <kanakshilledar111@protonmail.com>