Add "unknown" to LINUX_ABI

[exFalso]

This PR adds unknown to the LINUX_ABI array in build.rs. This is specifically to support --target=x86_64-fortanix-unknown-sgx, this target currently fails to link altogether because of the missing object files. With unknown added, everything works as expected.

[briansmith]

Obviously every "unknown" OS isn't guaranteed to have the same ABI as Linux. A more precise change to build.rs is needed to support SGX.

[exFalso]

Just noting: in SGX there is no operating system, it's all straight x86 code.

Also, it turned out that even with the project linking, at runtime the C code faulted, so we ended up completely replacing the ring dependency.

[briansmith]

Also, it turned out that even with the project linking, at runtime the C code faulted, so we ended up completely replacing the ring dependency.

That's because SGX requires its own implementation of the CPU feature detection in ring::cpu::intel. See the discussion in #775.

[briansmith]

Just noting: in SGX there is no operating system, it's all straight x86 code.

It is true that for SGX targets, "unknown" is the target OS, and this assembly code can be (AFAICT) used in SGX.

However, it isn't true that just because the OS is "unknown" then the assembly code is guaranteed to work. Consider x86_64-unknown-none, for example; this assembly code doesn't work for that target because it uses SIMD instructions, but SIMD instructions aren't (in general) available in x86_64-unknown-none, even if CPUID says they are supported.

[exFalso]

Makes sense, fair enough, thank you for the context!