[lib] Make lib compatible with -Wfall-through excepting legacy

Switch to a macro `ZSTD_FALLTHROUGH;` instead of a comment. On supported
compilers this uses an attribute, otherwise it becomes a comment.

This is necessary to be compatible with clang's `-Wfall-through`, and
gcc's `-Wfall-through=2` which don't support comments. Without this the
linux build emits a bunch of warnings.

contrib/linux-kernel/test/include/linux/compiler.h

@@ -18,4 +18,8 @@
 #define noinline __attribute__((noinline))
 #endif
 
+#ifndef fallthrough
+#define fallthrough __attribute__((__fallthrough__))
+#endif
+
 #endif