ld: Add lib32 directories for 32-bit emulation on FreeBSD/amd64

ld: Add lib32 directories for 32-bit emulation on FreeBSD/amd64

GNU ld currently fails to link 32-bit executables on FreeBSD/amd64 when
the linked libraries have dependencies on shared objects themselves:

$ gcc -m32 -o ei ei.c -lexecinfo
/var/gcc/binutils/amd64/lib/gcc/amd64-pc-freebsd14.0/13.2.0/../../../../amd64-pc-freebsd14.0/bin/ld:
warning: libelf.so.2, needed by /usr/lib/../lib32/libexecinfo.so, not found
(try using -rpath or -rpath-link)
/var/gcc/binutils/amd64/lib/gcc/amd64-pc-freebsd14.0/13.2.0/../../../../amd64-pc-freebsd14.0/bin/ld:
/usr/lib/../lib32/libexecinfo.so: undefined reference to `elf_begin@R1.0'
[...]

Fixed by handling FreeBSD/amd64 like Linux/x86.

Tested on amd64-pc-freebsd14.0.