cmake: rename construct_add_custom_command_for_linker_pass macro

Now that we avoid the two-step procedure, we can simplify the name of
the construct_add_custom_command_for_linker_pass macro.

Move description comment up to head of definition, making the purpose
clearer.

No functional change expected.

Signed-off-by: Mark Ruvald Pedersen <mped@oticon.com>