irq: Fix IRQ vector table relocation

The generation of the software ISR table and the IRQ vector table
(respectively generated by CONFIG_GEN_SW_ISR_TABLE and
CONFIG_GEN_IRQ_VECTOR_TABLE) should (in theory) go through three stages:

1. A placeholder table is generated in arch/common/isr_tables.c and
   placed in an orphaned .gnu.linkonce.{irq_vector_table, sw_isr_table}
   section

2. The real table is generated by arch/common/gen_isr_tables.py (creating
   the build/zephyr/isr_tables.c file)

3. The real table is un-orphaned by moving it in a proper section with a
   proper alignment

While all the steps are done automatically for the software ISR table,
for the IRQ vector table each architectures must take care of modiying
its own linker script to place somewhere the generated IRQ vector table
(basically step 3 is missing).

This is currently only done for 2 architectures: Cortex-M (ARMv7) and
ARC. But when another architecture tries to use the IRQ vector table,
the linker complains about that. For example:

  Linking C executable zephyr/zephyr.elf
  riscv64-zephyr-elf/bin/ld.bfd: warning: orphan section
    `.gnu.linkonce.irq_vector_table' from
    `zephyr/CMakeFiles/zephyr_final.dir/isr_tables.c.obj' being placed in
    section `.gnu.linkonce.irq_vector_table'

In this patch we introduce a new CONFIG_ARCH_IRQ_VECTOR_TABLE_ALIGN to
support the architectures requiring a special alignment for the IRQ
vector table and we also introduce a way to automatically place the IRQ
vector table in place in the same way it is done for the ISR software
table.

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