dhrystone: pass HZ=100 to fix build issue on musl

Fixes:
http://autobuild.buildroot.net/results/1b6cb1b40431bf484db2d99bfdc8237cbae6a9d7/
http://autobuild.buildroot.net/results/20e669d6e4bc3d872a355ef658ddad8f34676624/



musl doesn't provide sys/param.h to figure out the value of HZ, but as we
know we are building for Linux, and on Linux the clock_t values returned
from times(2) are defined in terms of USER_HZ which is always 100, we can
just as well tell dhrystone to use that directly.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>