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:		0f61170 ("libssp_nonshared: use only on i386 and ppc")
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit 9aa786c)

share/mk/src.libnames.mk

@@ -382,6 +382,8 @@ _DP_ztest=	geom m nvpair umem zpool pthread avl zfs_core spl zutil zfs uutil icp
 # The libc dependencies are not strictly needed but are defined to make the
 # assert happy.
 _DP_c=		compiler_rt
+# Use libssp_nonshared only on i386 and power*.  Other archs emit direct calls
+# to __stack_chk_fail, not __stack_chk_fail_local provided by libssp_nonshared.
 .if ${MK_SSP} != "no" && \
     (${MACHINE_ARCH} == "i386" || ${MACHINE_ARCH:Mpower*} != "")
 _DP_c+=		ssp_nonshared