test: arm_irq_vector_table: Fix when !CONFIG_GEN_IRQ_VECTOR_TABLE

This test is based on the wrong assumption that the IRQ vector table
symbols are always placed by the linker script also when
CONFIG_GEN_IRQ_VECTOR_TABLE is not actually set.

This is of course broken with the reworked mechanism of the IRQ vector
table placement.

Fix the test by reintroducing the old behaviour by forcefully
reinstating the symbols in the linker script.

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