Use -L to specify compat32 library paths instead of -Y

-Y is an uncommon linker option that is rather similar to -L. In
discussion with Peter it seems early amd64 development might have
required the -Y-specific behaviour, but it is no longer necessary.

Switch to -L which is more widely supported and much more commonly
used, to make it easier to link the FreeBSD base system with linkers
other than ld.bfd.

Submitted by: Rafael Ávila de Espíndola
Differential Revision: https://reviews.freebsd.org/D6681