include/ruby/3: Skip RUBY3_UNREACHABLE_RETURN for icc

I'm unsure why, but it fails on icc.

https://rubyci.org/logs/rubyci.s3.amazonaws.com/icc-x64/ruby-master/log/20200408T220004Z.log.html.gz
```
In file included from ../../.././include/ruby/3/core.h(32),
                 from ../../.././include/ruby/ruby.h(28),
                  from cxxanyargs.cpp(1):
../../.././include/ruby/3/core/rtypeddata.h(163): error: return value type does not match the function type
            RUBY3_UNREACHABLE_RETURN(NULL);
```

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