Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: haps_hs_defconfig arc):

arch/arc/kernel/unwind.c: In function ‘read_pointer’:
./include/linux/compiler.h:328:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
  do {        \
     ^
./include/linux/compiler.h:338:2: note: in expansion of macro ‘__compiletime_assert’
  __compiletime_assert(condition, msg, prefix, suffix)
  ^~~~~~~~~~~~~~~~~~~~
./include/linux/compiler.h:350:2: note: in expansion of macro ‘_compiletime_assert’
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
  ^~~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
 #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg)
                                     ^~~~~~~~~~~~~~~~~~
./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
  BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
  ^~~~~~~~~~~~~~~~
arch/arc/kernel/unwind.c:573:3: note: in expansion of macro ‘BUILD_BUG_ON’
   BUILD_BUG_ON(sizeof(u32) != sizeof(value));
   ^~~~~~~~~~~~
arch/arc/kernel/unwind.c:575:2: note: here
  case DW_EH_PE_native:
  ^~~~

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>