interrupts: use weak linkage instead of linkonce

.gnu.linkonce is an internal undocumented ld feature.
Just use __weak, which does the same thing we want.

This is only done for _sw_isr_table. _irq_vector_table
is left alone due to unwanted interactions between
__weak and the ld KEEP() directive.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>