[libomptarget][nfc] Introduce SHARED, ALIGN macros

Summary:
[libomptarget][nfc] Introduce SHARED, ALIGN macros
Move remaining cuda attributes behind such macros

Reviewers: ABataev, jdoerfert, grokos

Reviewed By: ABataev

Subscribers: openmp-commits, jvesely

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D71076