test/linkage-visibility: Ignore on musl targets
DynamicLibrary uses libc's dlsym() function internally to find symbols.
Some implementations of dlsym(), like musl's, only look at dynamically-
exported symbols, as found in shared libraries. To also export symbols
from the main executable, pass --export-dynamic to the linker.
(Plus see [here](https://stackoverflow.com/questions/4184017) and [here](https://stackoverflow.com/questions/6121838) for examples of where this is necessary on glibc as well.)
Add powerpc- and powerpc64-unknown-linux-musl targets
Add targets for musl on 32-bit and 64-bit powerpc. This requires some ABI fixes, as musl [uses the ELFv2 ABI on regardless of endianness](http://git.musl-libc.org/cgit/musl/tree/configure?id=8084d6ab57cdb0b8f328d3cdbad3b9d09eaaee04#n638). At the moment, powerpc64 support requires [an LLVM patch](https://reviews.llvm.org/D52013) to select the correct ABI; or I can add [a patch to Rust's LLVM backend](e8eaa2afd5) to always choose the right ABI.
Both architectures are able to run an extended bootstrap, and with some test fixes (e.g. #55561), there are no architecture-dependent test failures on powerpc64 (most failures in `src/test` are existing musl-host-related issues).
std: Add debugging for a failing test on appveyor
I'm not sure why this is failing, so let's hopefully get some more
information to help investigation!
rustc_codegen_llvm: traitification of LLVM-specific CodegenCx and Builder methods
This PR is the continuation of #54012 and earlier PRs, in the grand plan of #45274 to allow for multiple codegen backends.
High-level summary: interpose a set of traits between Rust's codegen logic and the LLVM APIs, allowing another backend to implement the traits and share most of the codegen logic. These traits are currently somewhat LLVM-specific, but once this refactoring is in place, they can evolve to be more general.
See [this README](756f84d7ce/src/librustc_codegen_ssa/README.md) for a writeup on the current trait organization.
