bitset: replace zero-length array with flexible-array member

Zero-lenght arrays are GNU C extension.
There's ISO C99 flexible array member, which
is preffered mechanism to declare variable-length types.

Flexible array member allows us to avoid applying sizeof
operator cause it's incomplete type, so it will be an error
at compile time. There're any moments else why it's better
way to implement such structures via FAM:
https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

In this issue it fixed gcc 10 warning:
"warning: writing 1 byte into
a region of size 0 [-Wstringop-overflow=]"

Closes #4966
Closes #5564