Add O3 to gcc linker

It is unknown if gcc uses the O3 optimization levels passed to the
compilation of individual cpp files, or if it needs O3 passed to the
linker as well.

The hash of the final binaries are different, thus O3 is passed to the
linker.

build_scripts/qt/compilers/gcc.pri

@@ -39,3 +39,4 @@ QMAKE_CXXFLAGS += -Wzero-as-null-pointer-constant
 
 # Qmake will append this even though higher optimization levels are used.
 QMAKE_LFLAGS_RELEASE -= -Wl,-O1
+QMAKE_LFLAGS += -Wl,-O3