avoid AltiVec keyword vector

avoid AltiVec keyword vector

The AltiVec header file is defining "vector", except in C++ build.
The keyword "vector" may conflict easily.
As a rule, it is better to use the alternative keyword "__vector",
so we will be able to #undef vector after including AltiVec header.

Later it may become possible to #undef vector in rte_altivec.h
with a compatibility breakage.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Christensen <drc@linux.vnet.ibm.com>