[Hexagon] Use default attributes for intrinsics

This switches Hexagon intrinsics to use the default attributes
(nosync, nofree, nocallback and willreturn). Especially willreturn
is needed to prevent optimization regressions in the future.

The only intrinsics I've excluded here are the load/store locked
intrinsics, which presumably aren't nosync.

Differential Revision: https://reviews.llvm.org/D137623