[lld-macho] Fix map file test on 32 bit hosts

The test added in https://reviews.llvm.org/D137368 has been failing
on our 32 bit arm bots:
https://lab.llvm.org/buildbot/#/builders/178/builds/3460

You get this for the strings:
<<dead>> 0x883255000000003 [ 10] literal string: Hello, it's me
Instead of the expected:
<<dead>>  0x0000000F  [  3] literal string: Hello, it's me

This is because unlike symbols whose size is a uint64_t, strings
use a StringRef whose size is size_t. size_t changes size between
32 and 64 bit platforms.

This fixes the test by using %z to print the size of the strings,
this works for 32 and 64 bit.