Add FreeBSD x86 support

[jbeich]

Fixes #3781

$ cargo test --target i686-unknown-freebsd --package wasmer-vm
[...]
running 15 tests
test mmap::tests::test_round_up_to_page_size ... ok
test table::table_element_size_test ... ok
test vmcontext::test_vmdynamicfunction_import_context::check_vmdynamicfunction_import_context_offsets ... ok
test vmcontext::test_vmcaller_checked_anyfunc::check_vmcaller_checked_anyfunc_offsets ... ok
test vmcontext::test_vmglobal_definition::check_vmglobal_definition_alignment ... ok
test vmcontext::test_vmfunction_import::check_vmfunction_import_offsets ... ok
test vmcontext::test_vmglobal_definition::check_vmglobal_definition_offsets ... ok
test vmcontext::test_vmglobal_definition::check_vmglobal_begins_aligned ... ok
test vmcontext::test_vmglobal_import::check_vmglobal_import_offsets ... ok
test vmcontext::test_vmmemory_definition::check_vmmemory_definition_offsets ... ok
test vmcontext::test_vmmemory_import::check_vmmemory_import_offsets ... ok
test vmcontext::test_vmshared_signature_index::check_target_shared_signature_index ... ok
test vmcontext::test_vmshared_signature_index::check_vmshared_signature_index ... ok
test vmcontext::test_vmtable_definition::check_vmtable_definition_offsets ... ok
test vmcontext::test_vmtable_import::check_vmtable_import_offsets ... ok

test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

   Doc-tests wasmer-vm

running 1 test
test src/libcalls.rs - libcalls (line 25) ... ignored

test result: ok. 0 passed; 0 failed; 1 ignored; 0 measured; 0 filtered out; finished in 0.00s

[Onceagain12]

Summit