Guard init_priority attribute within libc++

Not all platforms support priority attribute. I'm moving conditional definition of this attribute to `include/__config`.

Reviewed By: #libc, aaron.ballman

Differential Revision: https://reviews.llvm.org/D91565

GitOrigin-RevId: 2c7e24c4b6893a93ddb2b2cca91eaf5bf7956965
3 files changed