android/support: Fix long double support.

The code in src/math_support.c assumed that 'long double' and
'double' are actually the same size, but this is only true on
32-bit Android systems. For Android/64, 'long double' is actually
16 bytes / 128 bits on all CPU architectures.

Moreover, all Android/64 math libraries provide the missing functions,
so just ensure they are only compiled when targetting 32-bit systems.

+ Add a new unit test (math_unittest.cc)

Change-Id: Ie2b6388c60b208f928b57bf044f6b145c29bbcd0