[libomptarget][amdgpu] use --allow-shlib-undefined to link on FreeBSD

On FreeBSD, the `environ` symbol is undefined at link time for shared
libraries, but resolved by the dynamic linker at runtime. Therefore,
allow the symbol to be undefined when creating a shared library, by
using the `--allow-shlib-undefined` linker flag, instead of `-z defs`
(a.k.a `--no-undefined`).

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D107698