feat(libc): add support for fallthrough statement

Modern C compilers warn about unannotated switch/case fallthrough code,
and require either a comment with some magic words, or an explicit
compiler attribute.
Since some TF-A static analysis CI check suggests having a "fallthrough;"
statement instead of a comment, introduce a macro that implements that
statement, and emits the proper compiler attribute.

Change-Id: Ib34e615fb48d0f4a340aabfad4472e08d5c70248
Signed-off-by: Andre Przywara <andre.przywara@arm.com>