src.libnames.mk: add a comment explaining libssp_nonshared

libssp_nonshared is a special case for (only) i386 and power*. Add a
comment explaining why, based on the original commit message that added
it.

MFC after: 1 week
Fixes: 0f61170882cb ("libssp_nonshared: use only on i386 and ppc")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 9aa786cbb50e997739f31094dc8d45260f19c63c)