linker: update section names to be unambiguous

Recently 0ae0c3dc allowed for three digit priorities, this resulted
in objects potentially matching multiple sections, for example:

.z_init_PRE_KERNEL_2_0_0_
.z_init_PRE_KERNEL_2_?_*
.z_init_PRE_KERNEL_2_???_*

This does not seem to be detected by ld, but the IAR linker emits a
warning.

Add some extra qualifiers in the object section name to make it
unambiguous, this has the extra value of making it easier to interpret,
for example going from:

.z_init_POST_KERNEL_90_00012_

to

.z_init_POST_KERNEL_P_90_SUB_00012_

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>