flashbench: force gnu99 dialect on sh2a

Fixes:
http://autobuild.buildroot.org/results/33880036054c39cb04dad71bc8c0a9eece16fac6



CodeSourcery toolchains for sh2a only provide LLONG_MAX when
used with the gnu99 dialect.

Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>