Additional ARMA_64BIT_WORD support

[eddelbuettel]

ARMA_64BIT_WORD is useful for extended ranges and longer indices beyond the scope of a 32-bit integer. A reverse-dependency check is revealing that we cannot enforce it as a few dozen packages fail.

[eddelbuettel]

Setting ARMA_64BIT_WORD turns ints into long long which cannot map (as easily) back to R. Packages using integer vectors in Armadillo (ie arma::ivec) then have issues. A reverse-dependency check revealed a few dozen affected packages. While this is only a small subset of the well over 1000 packages using RcppArmadillo it still means we cannot easily or automatically switch this and need to keep it as an opt-in for users.