loader: Avoid -Wpointer-to-int cast warnings for Arm and RISC-V

On RISC-V, Clang warns with:

cast to smaller integer type 'unsigned int' from 'void (*)(void *)'

Instead, use %p as the standard format specifier for printing pointers.
Whilst Arm's pointer size is the same as unsigned, it's still cleaner to
use the right thing there too.

Reviewed by: brooks (mentor), emaste
Approved by: brooks (mentor), emaste
Differential Revision: https://reviews.freebsd.org/D25718