GccToolchain: Move reinterpretOptions into the header

Considering that this is a Noop by default we want that inlined.

Change-Id: Iefd822ad9e5fc576fb538d9b891f6e3ed3b914aa
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
GccToolChain: Make macro retrieval more explicit

Make the retrieval of information on pre-defined macros more explicit
and use the information to populate the tool chain with after it is
constructed.

This avoids doing one run of the compiler per auto-detected debugger during
the start-up of Qt Creator.

Change-Id: Ie741dccede91079b4126056676c31975f340dbae
Reviewed-by: Daniel Teske <daniel.teske@digia.com>