Nuttx GCC7: enable -Wimplicit-fallthrough

[jlecoeur]

Re-enable implicit fallthrough (with default level 3) warning in #8551

I tried various warning levels but they seem ineffective in the docker image.

• -Wimplicit-fallthrough=0 correctly disables the warning
• levels 1, 2 and 3 do not correctly accepts fallthrough comments
  See here for what the behaviour should be https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gcc/Warning-Options.html#Warning-Options.

I replaced the comments /* FALLTHROUGH */ by attributes [[fallthrough]] in C++ files, and by __attribute__((fallthrough)) in two C files.

[dagar]

This is a bit obnoxious given the "one compiler version only" discussion we just had, but can we do this without blatantly breaking old GCC (at least for a transition period)?

[jlecoeur]

Ahah that is a side effect I did not forecast!

Let's forget about this PR and try to fix the gcc install in the docker image.

(...or let's use a macro that checks the compiler version... no let's not do that)