Release 8.0.0

Update version file
Update release notes

update VERSION file to 7.0.0

update VERSION file to 6.0.0

update VERSION file to 5.2.0-mcs

update VERSION file to 5.2.0

update VERSION file to 5.1.0

update VERSION file to 5.0.0

update VERSION file to 4.0.0

add seL4_BenchmarkFlushCaches() for arm and ia32

Available when CONFIG_ENABLE_BENCHMARKS is set, flushes caches for
running cold-cache benchmarks

update VERSION file to 3.2.0