Remove strange default flags from iOS cpp module.

- visibility should not be hidden by default; this breaks shared libs.
- optimization, -dead_strip should not be platform specific.
- -g, -fPIE, -headerpad_max_install_names are already handled elsewhere.
- -fstrict-aliasing, -gdwarf-2 are already compiler defaults (redundant).
- -fexceptions has a task for a dedicated property (bad place anyways).
- -fpascal-strings, -fasm-blocks, user can set these if they want 'em.

Change-Id: I856d43f3d3539d0a0569c347615ef59dfc2f20f1
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Set better defaults for cpp.minimumOsxVersion and cpp.minimumIosVersion.

Now, each platform doesn't get both properties set, and the default is
usually the version of the SDK in use. This is both consistent with
Xcode's behavior and necessary to convince the tools that the target is
iOS Simulator and not OS X, as appropriate.

Change-Id: I63037138d38675c5f0cc53e721e42a4d7cc2c3b7
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>