MFC r314556-r314558

r314556:
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

r314557:
Correct an misunderstanding of MDSRCS.

MDSRCS it intended to allow assembly versions of funtions with C
implementations listed in MISRCS. The selection of the correct
machdep_ldis?.c for a given architecture does not follow this pattern
and the file should be added to SRCS directly.

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

r314558:
Garbage collect unused gdtoa related files on mips.

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