gcc 8.x is more pedantic than clang 7.x with format strings and the tests
passed `void*` variables while supplying `%s` (which is technically
incorrect).
Make the affected `void*` variables use `char*` storage instead to address
this issue, as the compiler will upcast the values to `char*`.
MFC after: 1 month
MFC with: r343362
Approved by: emaste (mentor; implicit)
Reviewed by: asomers
Differential Revision: https://reviews.freebsd.org/D18934
de00e09d82