Fix failing GC related tests with gcc in release builds

Fix failing GC related tests with gcc in release builds

Our conservative collector might is more likely to find old pointers
to QV4::Objects on the C stack in release builds, when gcc optimizes out
stores that seem unnecessary. These two tests are prone to that due to
temporary QV4::Values potentially ending up on the stack on x86,
because vmeProperty returns then through an invisible first argument that
is a pointer a a location on the stack.

This patch makes those temporaries explicit, clears them out after
usages and adds GCC pragmas to disable optimizations for these two
test functions.

Change-Id: Ie43841e869346792296911fee6fed80c745faeff
Reviewed-by: Lars Knoll <lars.knoll@digia.com>