kern.mk: drop flag only patched in-tree gcc understood

-mno-align-long-strings was a flag maintained by FreeBSD for the
now-deleted in-tree gcc. Upstream gcc has no such flag, so just drop
it.

The flag was originally submitted by bde and committed in 2002 (svn
r97911 & r104455). However, upstream gcc did address this same issue in
2004 (gcc svn r76694 / git 4137ba7ab7a), reducing long string alignment
in general, and to 1 with -Os.

Reviewed by: kib
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D27768