extensions.cmake: don't leak absolute paths in snippets-*.ld comment

Make headers look like this instead:

     /* From ${ZEPHYR_BASE}/arch/common/isr_tables.ld: */

Besides being shorter and more "private", this makes the content of
snippets-*.ld files the same no matter who built them and where.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>