Run 32-bit compat ldconfig on mips64.

mips64 supports COMPAT_FREEBSD32 (for o32 binaries), so run the 32-bit
compat ldconfig on it as well.

Reported by: brooks
Reviewed by: brooks, kib
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D17342