[rs6000] Consistently use '__vector' instead of 'vector'
Revision r265535 committed changes that used 'vector' instead of the
preferred '__vector'. There is a reason that '__vector' is preferred,
because it ensures no conflicts with C++ namespace. Indeed,
gcc/config/rs6000/xmmintrin.h undefines it, leading to errors:
gcc/include/xmmintrin.h:999:20: error: 'vector' undeclared (first use in this function); did you mean 'vec_or'?
gcc/include/xmmintrin.h:999:20: note: each undeclared identifier is reported only once for each function it appears in
gcc/include/xmmintrin.h:999:26: error: expected ')' before 'long'
gcc/include/xmmintrin.h:999:37: error: expected ')' before 'result'