Add attributes incidating intentional data type aliasing

Interpreting data of one type via a pointer of a different type is an
aliasing violating. This means that a compiler optimizer's analyzer
can assume that data loaded into an array as uint32_t isn't related
to data read out from the same array as uint64_t, and e.g. reorder
loads/stores.

Since these structs are intentionally used to load data via pointers
of a wrong size, tell the compiler that these accesses may alias
other reads.

This fixes the GetIntraPredictorTest tests of WelsI4x4LumaPredV_c
and WelsI4x4LumaPredH_c. (The compiler optimizer did the wrong thing
as long as WelsFillingPred8to16_c or WelsFillingPred8x2to16_c were
inlined into the calling function.)
Reenable GetIntraPredictorTest tests that were broken on android

These tests work as intended now.
Merge pull request #1254 from mstorsjo/alias-attribute

Add attributes incidating intentional data type aliasing, fix GetIntraPredictorTest on android