pcr: symbiflow: use fixed git revision
    
    Building this toolchain across the 3 supported architectures
    takes some time. So it's better to use fixed git revisions to
    ensure that all 3 architectures run the same code and not to
    have to update the pkgver just to build for another architecture.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>