Use -Oz when optimization=s in Clang

-Oz is the appropriate flag to use when you want to produce the smallest
possible binary, and is one would expect when setting optimization to s
or using the minsize build type.

mesonbuild/compilers/mixins/clang.py

@@ -40,7 +40,7 @@
     '1': ['-O1'],
     '2': ['-O2'],
     '3': ['-O3'],
-    's': ['-Os'],
+    's': ['-Oz'],
 }  # type: T.Dict[str, T.List[str]]
 
 class ClangCompiler(GnuLikeCompiler):

mesonbuild/interpreter/interpreter.py

@@ -2322,7 +2322,7 @@ def _warn_about_builtin_args(self, args: T.List[str]) -> None:
         # https://github.com/mesonbuild/meson/issues/3275#issuecomment-641354956
         # https://github.com/mesonbuild/meson/issues/3742
         warnargs = ('/W1', '/W2', '/W3', '/W4', '/Wall', '-Wall', '-Wextra')
-        optargs = ('-O0', '-O2', '-O3', '-Os', '/O1', '/O2', '/Os')
+        optargs = ('-O0', '-O2', '-O3', '-Os', '-Oz', '/O1', '/O2', '/Os')
         for arg in args:
             if arg in warnargs:
                 mlog.warning(f'Consider using the built-in warning_level option instead of using "{arg}".',