loader.efi: faults could try to print out call trace

with grab_faults, we can try to print out the trace of function calls.
Without symbol table, we can not translate addresses to function names,
but even addresses can help to track the bugs.

For loader functions, print out absolute address, so it could be
searched from objdump -d output.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35433