lib/string/strchr/: strrcspn(), stprcspn(): Add function and macro

lib/string/strchr/: strrcspn(), stprcspn(): Add function and macro

These APIs are to strrspn(), like strcspn() is to strspn().
They are like strcspn(3), but search from the end of the string.

The function is meant for internal use, and consistency with libc.
The macro is meant for normal use, since it returns a pointer,
which is what algorithms using this need.

See also strspn(3) and strcspn(3).

Signed-off-by: Alejandro Colomar <alx@kernel.org>