flashbench: unbreak build on microblaze

Fixes http://autobuild.buildroot.net/results/498287519fbb588ed4ee9624ff82002a4766a5f4



The old 4.1.2 binary microblaze toolchain only provides LLONG_MAX when
used with the gnu99 dialect.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>