msvc: Don't use /O3 with MSVC
MSVC doesn't support that option, and warns about ignoring an unknown
option.
@@ -299,7 +299,7 @@ msvc_optimization_args = {'0': [],
'g': ['/O0'],
'1': ['/O1'],
'2': ['/O2'],
- '3': ['/O3'],
+ '3': ['/O2'],
's': ['/O1'], # Implies /Os.
}
