Using FindThreads instead of hardcode -lpthread.

[RechieKho]

To fix #13114

From what I have read in iree_copts.cmake, if it is not targeting android and thread is enabled, link to pthread by default for all of the libraries. So I just link_libraries(Threads::Threads), assuming there is no libraries don't like accepting pthread and want to override the default. It compiles successfully on my machine.

Here is the generated compile command:

ccache /usr/bin/c++ -O3 -DNDEBUG -lm CMakeFiles/iree_samples_simple_embedding_simple_embedding_vmvx_sync.dir/device_vmvx_sync.c.o CMakeFiles/iree_samples_simple_embedding_simple_embedding_vmvx_sync.dir/simple_embedding.c.o -o ../../../../dist/misc/bin/simple_embedding_vmvx_sync  ../../../../dist/misc/lib/libiree_samples_simple_embedding_simple_embedding_test_bytecode_module_vmvx_c.a ../../../../dist/misc/lib/libiree_base_base.a ../../../../dist/misc/lib/libiree_hal_hal.a ../../../../dist/misc/lib/libiree_hal_drivers_local_sync_sync_driver.a ../../../../dist/misc/lib/libiree_hal_local_local.a ../../../../dist/misc/lib/libiree_hal_local_loaders_vmvx_module_loader.a ../../../../dist/misc/lib/libiree_modules_hal_hal.a ../../../../dist/misc/lib/libiree_vm_bytecode_module.a ../../../../dist/misc/lib/libiree_base_internal_fpu_state.a ../../../../dist/misc/lib/libiree_hal_utils_buffer_transfer.a ../../../../dist/misc/lib/libiree_hal_utils_deferred_command_buffer.a ../../../../dist/misc/lib/libiree_hal_utils_resource_set.a ../../../../dist/misc/lib/libiree_base_internal_arena.a ../../../../dist/misc/lib/libiree_base_internal_atomic_slist.a ../../../../dist/misc/lib/libiree_hal_utils_semaphore_base.a ../../../../dist/misc/lib/libiree_vm_bytecode_utils_utils.a ../../../../dist/misc/lib/libflatcc_parsing.a -lm ../../../../dist/misc/lib/libiree_hal_local_executable_loader.a ../../../../dist/misc/lib/libiree_hal_local_executable_environment.a ../../../../dist/misc/lib/libiree_modules_vmvx_vmvx.a ../../../../dist/misc/lib/libiree_base_internal_cpu.a ../../../../dist/misc/lib/libiree_builtins_ukernel_ukernel.a ../../../../dist/misc/lib/libiree_builtins_ukernel_arch_x86_64_x86_64.a ../../../../dist/misc/lib/libiree_builtins_ukernel_arch_x86_64_x86_64_avx2_fma.a ../../../../dist/misc/lib/libiree_builtins_ukernel_arch_x86_64_x86_64_avx512_base.a ../../../../dist/misc/lib/libiree_builtins_ukernel_arch_x86_64_x86_64_avx512_vnni.a ../../../../dist/misc/lib/libiree_modules_hal_utils_buffer_diagnostics.a ../../../../dist/misc/lib/libiree_modules_hal_types.a ../../../../dist/misc/lib/libiree_hal_hal.a ../../../../dist/misc/lib/libiree_base_internal_path.a ../../../../dist/misc/lib/libiree_vm_impl.a ../../../../dist/misc/lib/libiree_base_internal_synchronization.a ../../../../dist/misc/lib/libiree_base_base.a ../../../../dist/misc/lib/libiree_base_tracing.a -lpthread

Kind of a crazy big junk but the important thing is -lpthread is correctly placed at the back of the command, after libiree_base_internal_synchronization.a.

I also did not change anything regarding webassembly and riscv as I'm not sure whether FindThreads will also deal with linking for those two platform or not.

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[ScottTodd]

LGTM. Maybe @benvanik or @GMNGeoffrey could take a quick look also?

[GMNGeoffrey]

I know it's not necessary, but I think it would be nice to explicitly initialize thread deps to empty here, just so that we aren't expanding an undefined variable later. I think modern cmake may actually have a flag to error on undefined variables that we could turn on (IIRC we used to support cmake versions that were too old to enable this)

[GMNGeoffrey]

Hmm there's no way for me to enable auto-merge, but still allow you to fix my comment. To unblock things, I'm just going to turn on auto-merge and approve the CI run. If you see my comment before it completes, go ahead and add that, but otherwise it will just merge and I'll look into whether we can make cmake error on undefined variables