ruby: fix 'pcrel too far' build problem on SuperH architectures

The 'pcrel too far' problem detected in the autobuild on SuperH
architectures, seems to be caused by the -Os optimization flag. Using
standard optimization fixes the problem.

Fixes http://autobuild.buildroot.net/results/bc36e051e06f596c2fafdd3cc3745bb34b73ace3/



Investigated-by: Lionel Orry <lionel.orry@gmail.com>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>