[compiler-rt] Fix broken unittest related to 64-bit print format

Summary:
The printf is not working correctly on 64-bit MSVC.

The portable way to print "size_t" is to use "%zx" (size_t in hexa).

Reviewers: rnk

Subscribers: kubabrecka, llvm-commits, wang0109, chrisha

Differential Revision: https://reviews.llvm.org/D22588

llvm-svn: 276179