Support x86 (i.e. 32-bit)

[ojeda]

While x86 (i.e. 32-bit) will become less important over time, it is a key architecture of the kernel (and the original one, too). In addition, it is a non-64 bit target (which helps widening our tests to uncover potential issues) and it is a target that pretty much everyone can natively run. Thus we should support it.

Linux x86 targets with official Ubuntu binutils etc. support:

Linux arch  QEMU target     LLVM target  Ubuntu binutils            Example Rust triple
----------  --------------  -----------  -------------------------  ----------------------

x86         i386-softmmu    X86          binutils-i686-linux-gnu    i686-unknown-linux-gnu
x86         x86_64-softmmu  X86          binutils-x86-64-linux-gnu  x86_64-linux-kernel