Static amd context holder
Static amd context holder

@neapel hope this compiles, don't have an amd device or amdfft to test.

Poulpy21/vexcl,kapadia/vexcl,Poulpy21/vexcl,kapadia/vexcl,Poulpy21/vexcl,ddemidov/vexcl,ddemidov/vexcl