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);
```

include/ruby/3/core/rtypeddata.h

@@ -160,7 +160,7 @@ RTYPEDDATA_TYPE(VALUE obj)
 #if ! RUBY_NDEBUG
     if (RB_UNLIKELY(! RTYPEDDATA_P(obj))) {
         rb_unexpected_type(obj, RUBY_T_DATA);
-# if ! defined(__sun)
+# if ! defined(__sun) && ! RUBY3_COMPILER_IS(Intel)
         RUBY3_UNREACHABLE_RETURN(NULL);
 # endif
     }

include/ruby/3/memory.h

@@ -253,7 +253,7 @@ ruby3_size_mul_or_raise(size_t x, size_t y)
     }
     else {
         ruby_malloc_size_overflow(x, y);
-# if ! defined(__sun)
+# if ! defined(__sun) && ! RUBY3_COMPILER_IS(Intel)
         RUBY3_UNREACHABLE_RETURN(0);
 # endif
     }