Support arm (i.e. 32-bit)

[ojeda]

Armv7-A is becoming less popular given how cheap 64-bit chips are getting; however, we will have embedded 32-bit ARM around us for at least a decade, likely more. It is one of the key kernel architectures, too; and like x86, it is a good idea to support at least one 32-bit architecture to widen our tests and uncover possible issues etc.

Linux arm targets with official Ubuntu binutils etc. support:

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

arm         arm-softmmu      ARM          binutils-arm-linux-gnueabihf  arm-unknown-linux-gnueabi
arm64       aarch64-softmmu  AArch64      binutils-aarch64-linux-gnu    aarch64-unknown-none