Correct MDSRCS use in <arch>/string/Makefile.inc.

• Remove .c files which duplicate entries in MISRCS.
• Use the same, less merge conflict prone style in all cases.
• Use MDSRCS for mips (.c and .S files both ended up in SRCS).
• Remove pointless sparc64 Makefile.inc.
• Remove uninformative foreign VCS ID entries.

Reviewed by: emaste, imp, jhb
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D9841