arch: Fixes potential mangled isr table when SHARED_INTERRUPTS is enabled

The linker was optimizing away z_shared_isr() the zephyr_pre0.elf image
since the function is not used. However, zephyr.elf does use this
function via isr_tables.c file, generated after zephyr_pre0.elf.

This difference caused a shift in all symbol addresses by the size of the
z_shared_isr() function. isr_tables.c had pointers for zephyr_pre0.elf.
The build system assumes the ISR addresses are the same between both .elf
files, when in fact, before this fix, they were not.

When run on a device, interrupts were calling non-interrupt functions.

Signed-off-by: Dave Desrochers <dave@intercreate.io>