Support nix::sys::ptrace::sysemu on musl

[cvengler]

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This commit implements support for PTRACE_SYSEMU and PTRACE_SYSEMU_SINGLESTEP for x86 and x86_64 musl builds. In the future, this could even be extended to other architectures, for which musl provides support, such as 390x, powerpc, ...

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This pull request is still a draft and CI will fail. We need to wait for the next release of the libc crate, which includes support for it: https://github.com/rust-lang/libc/pull/3245