fixed abs() compilation for Android classpath on OSX

[bigfatbrowncat]

Trying to build Android class path (avian-pack) on OS X. Having a problem:

src/classpath-android.cpp:1209:10: error: taking the absolute value of unsigned type 'uintptr_t' (aka 'unsigned long') has no effect [-Werror,-Wabsolute-value]
  return llabs(arguments[0]);
         ^
src/classpath-android.cpp:1209:10: note: remove the call to 'llabs' since unsigned values cannot be negative
  return llabs(arguments[0]);
         ^~~~~
src/classpath-android.cpp:1215:22: error: using integer absolute value function 'abs' when argument is of floating point type [-Werror,-Wabsolute-value]
  return floatToBits(abs(bitsToFloat(arguments[0])));
                     ^
src/classpath-android.cpp:1215:22: note: use function 'std::abs' instead
  return floatToBits(abs(bitsToFloat(arguments[0])));
                     ^~~
                     std::abs
src/classpath-android.cpp:1215:22: note: include the header <cmath> or explicitly provide a declaration for 'std::abs'
2 errors generated.

This is the solution. Although we are not linking against standard library, we are using abs and llabs from it. Additionally i've fixed a nasty type conversion bug.

[bigfatbrowncat]

That's strange... "Trace" test failed, but I changed nothing except classpath-android.cpp, so that shouldn't be my fault...

[dicej]

@bigfatbrowncat it's probably not your fault. See issue #429

[bigfatbrowncat]

Fixed code just as you proposed. Thanks!