Always link with --gc-sections

Debug builds don't link with --gc-sections, which can cause link failures when translation units built with RTTI pull in inline constructors for classes that derive from classes whose constructors are defined in translation units built without RTTI, even if those classes aren't instantiated.

Eg:

obj/qt/core/libOxideQtCore_public.a(obj/qt/core/api/OxideQtCore_public.oxideqglobal.o):oxideqglobal.cc:typeinfo for base::DelayTimer: error: undefined reference to 'typeinfo for base::Timer'.

base::DelayedTimer has an inline constructor and derives from base::Timer, whose constructor is defined in another translation unit. However, oxideqglobal.cc never instantiates base::DelayedTimer (it does indirectly pull in base/timers/timer.h).

Linking with --gc-sections fixes this