Fix two errors when cross-compiling with Vala

- AttributeError: 'ValaCompiler' object has no attribute 'get_program_dirs'

  Fixed by adding a `get_program_dirs()` function to the base Compiler
  class, to match `get_library_dirs()`

- KeyError: 'vala_COMPILER'

  Fixed by creating the Vala compile rules for all machines, not just
  the build machine.

mesonbuild/backend/ninjabackend.py

@@ -1653,8 +1653,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
                 self.generate_cs_compile_rule(compiler)
             return
         if langname == 'vala':
-            if self.environment.machines.matches_build_machine(compiler.for_machine):
-                self.generate_vala_compile_rules(compiler)
+            self.generate_vala_compile_rules(compiler)
             return
         if langname == 'rust':
             self.generate_rust_compile_rules(compiler)

mesonbuild/compilers/compilers.py

@@ -1117,6 +1117,9 @@ class Compiler:
     def get_library_dirs(self, *args, **kwargs):
         return ()
 
+    def get_program_dirs(self, *args, **kwargs):
+        return ()
+
     def has_multi_arguments(self, args, env) -> Tuple[bool, bool]:
         raise EnvironmentException(
             'Language {} does not support has_multi_arguments.'.format(