boost.simd: toints - avoid static variables

according to our observations, static variables seem to harm the performance on clang/libcxx, as the compiler generates code to guard the initialisation of  the static variables (`__cxa_guard_acquire` and the like). avoiding the `static` keyword improves the generated code by avoiding the function call which has a measurable impact on the generated code.

for now we only came across this issue in this specific location, but it would not surprise me, if other parts of boost.simd/nt2 have similar issues.