C# and Java compilers should have `is_cross = False`

All compilers should define this attribute. Probably should change base
class to require this.

mesonbuild/compilers/cs.py

@@ -32,6 +32,7 @@ class CsCompiler(Compiler):
         self.language = 'cs'
         super().__init__(exelist, version)
         self.id = id
+        self.is_cross = False
         self.runner = runner
 
     def get_display_language(self):

mesonbuild/compilers/java.py

@@ -23,6 +23,7 @@ class JavaCompiler(Compiler):
         self.language = 'java'
         super().__init__(exelist, version)
         self.id = 'unknown'
+        self.is_cross = False
         self.javarunner = 'java'
 
     def get_soname_args(self, *args):