Fix regression caused by libbacktrace API change.

The stack dump was not printing leading zeros for data after the
change to remove uintptr_t types from the libbacktrace API.

Bug: 65682279

Test: Created an arm tombstone and an arm64 tombstone and verified
Test: that the stack data has leading zeros.
Change-Id: I1fbec2c4fa7c8b0fab18894c5628d18c5a580299