rtld: Add UTRACE_LOAD_OBJECT traces for rtld and the main binary

These two objects are loaded by the kernel not rtld, but adding these
two traces means that UTRACE_LOAD_OBJECT traces now describe the
mappings for all executables and DSOs in a process' address space.

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: AFRL, DARPA
Differential Revision: https://reviews.freebsd.org/D52034