linker: common-rom: Keep z_shared_isr regardless of dynamic interrupts

Currently, the z_shared_isr symbol is only kept if
CONFIG_DYNAMIC_INTERRUPTS=n. Whenever CONFG_DYNAMIC_INTERRUPTS
is set to 'y', said symbol gets dropped when zephyr_pre0.elf
is created. This is wrong because dropping/keeping z_shared_isr
shouldn't be influenced by enabling/disabling support for the
dynamic interrupts.

This commit fixes the aforementioned issue by removing the
dependency on CONFIG_DYNAMIC_INTERRUPTS.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>