objc/objcpp compiler: accept all gnu stds corresponding to c/c++ stds

The clang compiler now reimplements and re-checks the c_std and cpp_std
options in order to use them for objc as well, but it didn't
consistently support the same options. First it completely excluded all
the gnu ones, and then it added a handful of them but not for C++.

Be fully consistent -- or at least as consistent as we can be, given a
minimally working fix. (The C/C++ compiler mixin actually gates
different stds depending on detected clang version, we do not do that
here.)

Fixes regression in c54dd63547
Fixes incomplete fix from #8766 (which didn't fix objcpp at all)

Fixes #9237

mesonbuild/compilers/objc.py

@@ -90,7 +90,7 @@ class ClangObjCCompiler(ClangCompiler, ObjCCompiler):
         opts.update({
             OptionKey('std', machine=self.for_machine, lang='c'): coredata.UserComboOption(
                 'C language standard to use',
-                ['none', 'c89', 'c99', 'c11', 'c17', 'gnu99', 'gnu11'],
+                ['none', 'c89', 'c99', 'c11', 'c17', 'gnu89', 'gnu99', 'gnu11', 'gnu17'],
                 'none',
             )
         })

mesonbuild/compilers/objcpp.py

@@ -91,7 +91,7 @@ class ClangObjCPPCompiler(ClangCompiler, ObjCPPCompiler):
         opts.update({
             OptionKey('std', machine=self.for_machine, lang='cpp'): coredata.UserComboOption(
                 'C++ language standard to use',
-                ['none', 'c++98', 'c++11', 'c++14', 'c++17'],
+                ['none', 'c++98', 'c++11', 'c++14', 'c++17', 'gnu++98', 'gnu++11', 'gnu++14', 'gnu++17'],
                 'none',
             )
         })