Correctly determine the real executable in crunched binaries

This should fix cases like su setting argv[0] to _su for /bin/sh.
Previously cheribsdbox (a crunched tool we use in CheriBSD to reduce the
size of our minimal disk images to allow loading them onto FPGAs without
waiting forever for the transfer) would complain about _su not being
compiled in, but now that we also look at AT_EXECPATH it correctly
invokes the sh tool.

Note: we use use AT_EXECPATH instead of the KERN_PROC_PATHNAME sysctl to get
the crunchgen binary name since it seems like KERN_PROC_PATHNAME just
returns the last cached path for a given hardlink.
When using su, instead of invoking /bin/csh this would invoke the last
used hardlink to cheribsdbox. This caused weird test failures when running
tests due to id being executed instead of echo:

$ id # id is a hardlink to /bin/cheribsdbox
$ su postgres -c 'echo 1' # su is also a hardlink
uid=1001(postgres) gid=1001(postgres) groups=1001(postgres)

Obtained from: CheriBSD

Reviewed By: emaste, brooks
Differential Revision: https://reviews.freebsd.org/D25998