Dalvik: Better compatibility with different compilers

This fixes building with clang, and possibly future versions of gcc:
- Don't error out on unused functions generated by gen-mterp.py
  even when we're using -Werror
- Eliminate some dead code (Word/kWordSize definition) clang warns about
- Add a return statement to a switch path causing a possible code path
  with uninitialized variables (if the compiler doesn't know what
  dvmAbort() is)
- Initialize a variable

Change-Id: I9a3c0a1ad4dce10445e2251e491f18511413ca50
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>