No longer able to detect exception model right

[Andrew Pinski]

After the update to configure in libstdc++ on openbsd (and any other 
target that uses sjlj by default),
configure will say the exception model is "call frame" all the time 
unless you use --enable-sjlj-exceptions
which is not right.
My testresults show that it broke almost of c++ compiling.


Thanks,
Andrew Pinski