linker: Fix _vector_end symbol location

The 219d5b5a commit is breaking the STM32F0 platforms.

The problem is that the PR is changing the location the _vector_end
symbol is pointing to. These platforms require the symbol to be pointing
to a location in memory after the IRQ vector table.

Fix the issue by introducing a new _vector_end symbol when the IRQ
vector table is present that points to the correct location.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>