Cross compiling for ARC processor (Argonaut RISC Core)

[jonbc]

I got Rust code to compile for the arc-elf32 target using rustc_codegen_gcc and using Heath123's rust-casio-docker repository. Modifications to rust-casio-docker:

• Had to modify rustc_codegen_gcc (applying a patch in the Dockerfile) to remove fminf, fmin, fmaxf, fmax from builtins in src/context.rs.
• Modified set_superh_flags.patch to only specify context.add_command_line_option("-mcpu=em4") (get the -mcpu flag from ARC-Options).
• Changed wrapped_linker to arc-elf32-gcc in sh-link-wrap.sh and changed \x2A to \x2D (since that's ARC's e_machine value)
• Changed linker in sh3eb-elf.json to arc-elf32-gcc.
• Changed --target in the Dockerfile to arc-elf32.

You can test the compiled Rust code using QEMU. Follow kolerov's synopsys repository on how to build QEMU for ARC.

Note that ./test.sh in rustc_codegen_gcc still fails.

• The first issue you'll experience is an arc_legitimize_tls_address issue.
• So remove the thread-local storage (TLS) related tests in example/mini_core.rs.
• The second issue is that arc-elf32-ld can't find libc. I haven't figured out a solution to that yet.