Add support for arm-unknown-linux-musleabi

[imlk0]

The changes included in this PR are intended to add support for the arm-unknown-linux-musleabi target to proot-rs.

The main changes are:

• Add conditional compilation statements for different architectures due to differences in system calls.
• Add missing parts for loader.c.
• ptrace(): added support for reading and writing register sets on arm target.
• Some additional fixes such as changes to the types of some variables.

[oxr463]

How can I build for android? I ran cargo build like usual, but it still created an x86_64 binary:

file ./target/debug/proot-rs
./target/debug/proot-rs: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, with debug_info, not stripped

[imlk0]

How can I build for android? I ran cargo build like usual, but it still created an x86_64 binary:

file ./target/debug/proot-rs
./target/debug/proot-rs: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, with debug_info, not stripped

For building for android, I basically followed the steps here:
https://blog.rust-lang.org/2016/05/13/rustup.html#example-running-rust-on-android

Unlike other Linux targets, to build for android you need to first download the NDK and unzip it somewhere. And then edit .cargo/config.toml and add something like the following. (We don't need make-standalone-toolchain.sh anymore, google says this has been deprecated)

[target.arm-linux-androideabi]
linker = "<path-to-ndk-dir>/toolchains/llvm/prebuilt/linux-x86_64/bin/armv7a-linux-androideabi18-clang"

The next steps are the same as for the other targets.
First add target arm-linux-androideabi to current rust toolchain:

rustup target add arm-linux-androideabi

Then specify the target we need to build for and start building

cargo build --target=arm-linux-androideabi

Unfortunately, it is not yet possible to successfully build proot-rs for android (the target arm-linux-androideabi). loader.c is one of the problem, and I am trying to solve it.

But for now we can build to the arm-unknown-linux-musleabi target, with the following commands:

rustup target add arm-unknown-linux-musleabi
cargo build --target=arm-unknown-linux-musleabi

The output will be located at target/arm-unknown-linux-musleabi/debug/proot-rs