Update comments and CFLAGS in sys/conf/kern.mk, introduced in r221879, to match reality: clang does _not_ disable SSE automatically when -mno-mmx is used, you have to specify -mno-sse explicitly.
Update comments and CFLAGS in sys/conf/kern.mk, introduced in r221879,
to match reality: clang does _not_ disable SSE automatically when
-mno-mmx is used, you have to specify -mno-sse explicitly.

Note this was the case even before r232894, which only makes a change in
the 'positive' flag case; e.g. when you specify -msse, MMX gets enabled
too.

MFC after:	1 week

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