sgx-lkl-musl code interleaves x86-64 and LKL code

[davidchisnall]

Currently, we have a lot of code paths in musl where we've replaced the normal x86-64 versions with LKL versions. We should eventually revisit these and try to clean them up so that we have a clean separation of upstream vs LKL code.

For example, lsds/sgx-lkl-musl#11 (my fault) replaced the assembly to call the clone syscall. We need to extend the build system to allow musl to select the correct version.

We also need to think carefully about code location. We are likely to have at least x86-64 and AArch64 versions of SGX-LKL in the long term, so we need to handle the differences between the two. We will likely want either lkl/{arch} or {arch}/lkl subdirectories for the LKL-specific code.

[bodzhang]

Does the work related to #403 address this issue?

[davidchisnall]

No, #403 makes this worse in the short term.

[bodzhang]

Should this issue be added to "Clean up layering" project and bumped to P1?

[davidchisnall]

No, it's not a layering violation, it just makes merging changes from upstream musl slightly harder.