Fix __gnu_Unwind_Find_exidx again.

The math on the size calculation was wrong as the type of
__exidx_start/__exidx_end was unsigned rather than a char. Make a
struct that represents each item instead and remove the division.

Test: built artifacts and used them in the NDK
Bug: None
Change-Id: Ic2c0c123a369b9319e8645d806d659290eb2f69c