Implement -Wimplicit-fallthrough (version 9)

[Gerald Pfeifer]

Hi Marek,

On Sat, 24 Sep 2016, Marek Polacek wrote:
> All right.  I'll commit the patch on Monday.

my thrice a week bootstrap on old (but still "supported") 
i?86-unknown-freebsd9 broke as follows:

  cc1plus: error: unrecognized command line option "-Wno-implicit-fallthrough"
  gmake[3]: *** [Makefile:1102: insn-emit.o] Error 1
  gmake[3]: *** Waiting for unfinished jobs....
  gmake[3]: Leaving directory '/scratch/tmp/gerald/OBJ-0927-0731/gcc'
  gmake[2]: *** [Makefile:4571: all-stage1-gcc] Error 2
  gmake[2]: Leaving directory '/scratch/tmp/gerald/OBJ-0927-0731'
  gmake[1]: *** [Makefile:24240: stage1-bubble] Error 2

(Also filed as https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77751 )

Gerald