[scudo] Read ARM feature bits using Fuchsia APIs.

Summary:
    Fuchsia uses zx_system_get_features in lieu of getauxval.
    
    Use this call when checking for CRC32 support.

Reviewers: cryptoad

Reviewed By: cryptoad

Subscribers: delcypher, llvm-commits, #sanitizers, kristof.beyls, chrib

Differential Revision: https://reviews.llvm.org/D45896

llvm-svn: 330598